Dating DAX Chairs (By Boot Glides?)  

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I cant say exactly but the x bases are older and generally you see far fewer of them the most popular are the black finish legs.The rubber sock glides are older as well...

I have never seen the boot gl...
I have never seen the boot glides on the chair at the right before. My guess is that these have been replaced at some point.
The glides of the chair on the left are more common. I have these glides on one of my rope-edge Zenith chairs, which could mean that these glides were used on the last of the first generation (rope-edge) chairs manufactured by Zenith and also on the first run of the 2nd generation Zenith chairs (without the rope-edge, but with the large pucks).
My chair also has the original Zenith paper tag (not the checkerboard label) hanging in a string from the x-base which I found quite amazing...

So what year would that be?
Okay very interesting! So would the first run of the 2nd generation Zenith chairs be early 1950s?

Yes, early 50's. I'm not...
Yes, early 50's. I'm not sure exactly which year exactly. Some people say Zenith started producing these chairs with the rope edge in 1949 and without it in 50-51, but I'm not really sure.
Maybe Lucifersum knows more, if he's around?

re: dating dax
so I just got an chair, off-white, supposedly kept in the basement for many years (estate): it has the dimensions of a dax, body seems like a knockoff (fibres hardly visible), but on the X base at the center it has the letter "D" you mention....

...continued
...or is it possible that this has the "finish" I read about, & what I have is an early DAX-1 with its naugahyde cover gone missing?

different DAX Glides, as...
different DAX Glides, as described in "modern classic" (a LuciferSum recommendation):

Boot glide
That type of glide dates to 1954 - 1957.
The other glides on the chairs pictured do look like non-HM replacements.

You want to date a DAX chair?
You want to date a DAX chair?

at least no strippers...
at least no strippers involved here.

Phase II
I want to guess that the large shockmounts are very early and definitely exclusive to Zenith. Zenith went through at least two phases of the chairs - this I think, is 1954 or slightly before. Earliest productions had the rope edge, middle production didnt, but later production had screw in glides. 1954 is a safe bet.

Replacement Glides
It's possible that the boot glides to the right may be replacement glides. My local private high school is a fine source for used Eames chairs, and they seem to come with a wide variety of replacement glides. Given that this institution has thousands of Eames chairs, I suspect that they order the replacement glides in bulk. I've also found vintage Domes of Silence on Ebay in over-the-counter packaging, and, they're available at my local hardware store. So, it's my opinion that the ones to the right are replacement.
The order of the generations of glides on the chart is curious, as the early Zeniths would have hollow ends, and the later Zenith and Venice/no-rope wide puck, would have boot glides. It's possible that the Venice location only got one set of tools. But, it would seem to me that the hollow end/tubular base would be an innovation, and come
later. Just my thoughts.
I did have a set of 3 side shells[2 Prachment on Black, 1 greige on Zinc] that had H-Base/Domes of Silence, and no wide mount molded provisions. So, it's leads me to think that some of the first groups that came out of Zeeland had Domes of Silence.