Paul McCobb Serving Cart for Calvin - HELP  

Travertine
I've had a couple of those, seen many more, and the tops were always travertine. It is easy enough to change the top, it just rests in the brass frame. So it very likely could have gotten damaged or broken and been replaced.

I love those carts! In fact...
I love those carts! In fact it was a 9203 serving cart that started my interest in Paul McCobb a few years back.
There were three showroom material choices for the tops of the 9203 Serving Carts
1) White Carrara Glass
2) Roman Travertine
3) Radio Black Marble
The Radio Black marble frequently had little flecks of color and even veins of color running through it and sometimes (according to Paul McCobb's daughter in an interview a while back) visible fossils in it.
The Irwin and Calvin lines were showroom furniture. Pretty much every piece of furniture sold by Directional during the 50's was a custom order. If a decorator wanted a certain type of marble or granite for a top that wasn't a stock item the showroom would do whatever they could to service the client.
http://paulmccobb.blogspot.com

More about Travertine
Travertine and/or Radio Black Marble were the cheaper options at $285 for the top alone, the Cararra Glass option was $305.50 and the cart itself with no top sold for $225.
(all prices taken from the July 1956 Directional price list)
http://paulmccobb.blogspot.com

Well, there you go.
Obviously, the carts I've seen were purchased by cheapskates who would not spring for the white glass.

Here are pictures
... of the two that I've had. Like I said, both were travertine. One had the usual finish on the wood, while the other had a lighter bleached mahogany which was very handsome and looked great with the travertine.
I think the warm textural travertine compliments the design well, whereas the white glass and black stone might be a bit stark or high-contrast in comparison.
Interestingly, a google image search only shows examples with light colored tops, none with the black stone. It appears the black top is much more uncommon. Don't know that that would effect the value at all, but it's interesting.

Thanks for all the input! I...
Thanks for all the input! I couldn't find any black examples online either - therefore my question. The Radio Black, as described above, sounds exactly like the one I'm looking at. The wood is bleached looking, and it does make for a rather high contrast.
Since you guys seem to know so much..... one more question: The casters are 'ball' type. Did these carts ever come like that? There's an incomplete example on 1stdibs with 'ball' casters....
Cheers!

No Balls
The images that Pegboard published both show the original casters.
I get the impression that ball casters did not come into being until the 60's, or at least they were not commonly used until the mid 60's.
Bottom line is that the ball casters are not original to the cart.
http://paulmccobb.blogspot.com