saarinen base markings  

Most authentic vintage Knoll tulip stools and chairs
will be marked "BR 51".

I don't know ....
....about that. I've got eight vintage Knoll Tulip chairs, six w/o arms and two with. They were bought in the mid to late '70s at Knoll in Frankurt, Germany. The only marking on the chairs, is "Knoll International" stamped/embossed on the bottom metal outer ring. "BR 51" is nowhere to be found.

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My old Knolll tulip stool has a BR51 stamped into the metal, also. I've never noticed before. Any ideas as to why?

chair vs. stool
Is it possible the chairs and the stools have different markings?

I said "most", not "all"....
I have seen BR51 on both chairs and stools, but my experience has been with vintage North American chairs. Production in Europe may have been by a different subcontractor.
You can see the BR51 in these pictures on both stools and chair, also see previous thread link at the bottom:
http://www.designaddict.com/design_addict/forums/index.cfm/fuseaction/th...

That link ...
... refers to in part to "BR51 stamp on base." The picture shows the stamp, but doesn't really show where it's stamped on the Tulip chair base, as it was so well shown on the Tulip stool, stamped on the bottom of the base.
Is it supposed to be stamped on that portion of the base just under the cushion? If yes, then let's have a picture showing that. Because right now there's nothing to see!

It is not under the cushion.
BR 51 is the part number of the base.
The same base was used on both the chair and the stool, i.e. the tops are interchangeable during assembly.
It is in the same place on the chair base, as the stool shown.
Here is another example, if you read the description, all bases are marked BR 51:
http://www.worthpoint.com/worthopedia/vintage-knoll-saarinen-mid-century...

Bario, don't worry I'm sure your chairs are legit.....
Euro production was likely subcontracted to European factory that did not use the same cast, or had a different system for labeling their parts.
Also, I don't know exactly when, but the BR 51 disappeared on later chairs.

Another note.
An aluminum base will have a slight chalky residue that forms on the
surface. I don't believe under the base was ever factory powder coated.
If it is painted underneath it may have been spray-bombed at some point.
(an owner DIY repair and not such a good thing)
My two tulips have BR 50 instead of BR 51. They both have the knoll sticker
on the seats between the velcro strips. Minty velcro so they must be new
though i bough them used.
My junk tulip, found on the street, has a dozen numbers and dashes followed
by ALCOA stamped in the bottom aluminum base. No sign of velcro residue.

Alcoa
I have an older Tulip armchair (non-swivel, vertical label) that has just Alcoa stamped on the bottom of the base.

BR 50 and BR 51
One is swivel, the other is stationary, though I can't remember which is which.

My BR 50's
swivel.

Purchased five
Thanks to all. I purchased all five. They are marked BR51 and swivel. Definitely aluminum. I paid less than $100 for each, so I am not too concerned if they are knock offs or not.
I did notice one is markedly different:
1. The bottom edge is straight, not tapered as the others are;
2. The seat cushion does not have "holes" that you can feel through the bottom fabric, whereas the other four do.
3. The top of the stem is slightly wider that the rest, by approximately .5".
It would be interesting to try to line up a history of the chairs and explore the differences in the mfg. process.

We've got dozens of tulip stools w/o a BR stamp on them
At work, we've got scores of these stools (some of which still have the Knoll labels) but they don't have the BR stamps on the bases. I'm sure that casting must have been discontinued at some time. I'm not sure when these were acquired...either in the late 60s or early 70s. They have all been reupholstered at sometime in the past. The bases are black and the undersides of the bases are black as well. The swivel/mounting mech is consistent with all of the other Knoll tulip bases I've seen.