Nakashima straight chairs.  

Also all of the spindles are shaped by hand for about the top 1/3 to 1/2 but machined on the bottom half. 

Possibly Knoll, not from New Hope.
Nice find.

They are indeed early version for Knoll by Nakashima.  Do not refinish or do any sanding on these chairs or you will loose all the hand tool marks that Nakashima or one of his workers left on these chairs.  They may appear crude and not as refined compared to the reissue studio like version by Mira Nakashima for Knoll recently.  The chairs are primarily birch (stained in various shades from natural, walnut and black) with hickory spindles.  Even though they were re-interpretaion of the traditional windsor chairs, they were also made using different woods as needed.  I found mine years ago at Goodwill for $14.00 (for four chairs). 
Also do not correct the perpendicular cuts on the bottom of the legs, Nakashima intended them that way to wear out flat eventually.
And for clarification purposes, Nakashima like other modern 20th century woodworkers, he did use power tools mostly for rough cutting or to speed things up a bit contrary to popular belief that everything he did were handmade.

Is it documented anywhere that Knoll N19's were produced by Nakashima or at New Hope, mini? I may be wrong, but I assumed Knoll's was a licensed design (that differs in significant details from those made by Nakashima) and was mass-produced elsewhere.

Any book on Nakashima talks about his short lived relationship with Knoll.  Nakashima wanted them to be crafty but Knoll at the time was focusing on the International Style and the Nakashima funiture line no longer fits the Knoll look.
The Soul of a Tree: A Master Woodworker Reflections by George Nakashima 
Right after leaving the interment camp (after WWII) I think even before moving to New Hope, he designed a 3 legged childs high chair for her daughter Mira which was the basis of the early N19 and other later studio chairs. 
Also, other variations of this chair exist out there. Not fakes or pirated copies but mabe by previous woodworkers who used to work for the studio of George Nakashima in the New Hope area.  There was an article a few years back about Mira Nakashima asking former staff/colleagues to stamp or label their work for identification purposes. 

This might help I hope to answer the question on who made the current versions for Knoll,
http://www.bgccraftartdesign.org/items/show/1
http://www.bgccraftartdesign.org/files/original/ae13ce2063954edba328c17b2531ec74.pdf
And no it is not a Daddy Dearest expose.
" MN: Yeah we were trying to do that with my son. But people nowadays associate Nakashima with what they call one-offs. Everything is individually designed and crafted and made specifically for them, so what we had thought would be a semi-manufactured line that my son would be in charge hasn’t taken off at all. We took it to Architectural Digest, we took it to Neocon last year and we’ve tried to promote it and it just hasn’t taken off. I mean we would like to be able to. There’s just the way these things are made there’s no fast way to make them. Knoll has taken that chair over there which is a straight back chair, and mass-produces that, but that’s the only one. And then there are knock-offs for the Conoid chair. They’re being made in Southeast Asia and they’re fairly inexpensive—so if you want a Nakashima knock-off—[laughs.]
MS: It has to be the real thing. [laughs.]
MN: The Knoll chair is actually true to the design, it’s just the manufacturing is different.
MS: So the material is different, the finish is different?
MN: The construction is different. Well the construction is basically the same but it’s put together by machine and it’s a catalytic varnish and the wood is steam dried and so it just looks different and feels different because it’s not handmade...."

Okay, so the chairs in question are possibly contemporary with, but neither Nakashima nor Knoll, then.
Apologies if my question caused any confusion.

Here is a pretty good article on just the straight chair.  The n19s were produced independtly of the New Hope studio, but were still finished by hand.  The new productions by Knoll are based on an original from New Hope.  So they vary even from the N19s from the 40s/50s and are closer to the the original design, but are all machine made.
http://www.themagazineantiques.com/news-opinion/current-and-coming/2009-...
edit:  "The early orders were made in Nakashima’s own studio, before production was moved to East Greenville. The line was discontinued in 1955 when Nakashima opted to produce and market all of his designs himself."   
From the Knoll site.  

The chairs in question are vintage examples of an early (unlabeled) straight chairs by Nakashima for Knoll.
This might help also.
http://www.themagazineantiques.com/news-opinion/current-and-coming/2009-05-26/celebrating-the-straight-chair-a-conversation-with-mira-nakashima/
http://www.knoll.com/product/straight-chair
I had seen other early examples in museum display/exhibition with no Knoll labels, some later ones did before being discontinued in the mid 1950's.
I also read somewhere that George Nakashima himself dislikes signing his work and gets irritated when customers ask him to sign the pieces he made.

I  wonder if there is any documentation of the straight chair being made by previous employee at the time of original production.  Seems unlikely that anyone would have gone to that extent when Knoll was making them.

Correction.  Just saw on the Knoll site that the early orders were produced at New Hope before they moved production to the factory.  Wish there was a way to date and actually verify these chairs.  Maybe I could try and contact the original consigner of the chairs, but that might be an awkard conversation.  

The other straight chair look alikes made by previous staff have less or more spindles or grouped in two's or three's and have different leg bracing location. They were not meant to deceive.  
It takes time and knowledge to be able to tell a real Nakashima from a look alike.
Not this chair but most museums now like the Indianapolis Museum of Art and Boston MFA have Nakashima examples that anyone could sit on and touch without getting thrown out of the museum or arrested.  If you have one near you, you should go and check it out.  It is fun to sit on a $22.000.00 or more Conoid bench for free. 
You had been gifted and I hope you will keep and enjoy these chairs for a very long time.  It is unfortunate that some only acquire these for labels, designer pedigree, investment and resale reasons.

Just to muddy the waters, I have seen vintage labeled (by another commercial maker) knock offs that were very hard to distinguish from originals.

Any chance you remember the name of the other maker?