Here is Fel-Pro's answer received this morning:

Hi Jon,

Unfortunately we do not offer that seal anymore. The tooling wore out and the volume was too low to justify the costs.

Best Gasket does offer a rubber RMB seal for this application, so a distributor of their products may be able to get that seal for you.

Thank you for contacting us.

On another note, I located 2 more of these (the Fel-Pro gaskets). I bought them and received them today. They came from Detroit Gasket, were in old boxes and one had been opened but not used. So at least at this point I have a few extras of the old Fel-Pro seal. Has anyone compared these to the Best Gasket rubber seal? I'd be interested to know the measurements and to see if they vary from Fel-Pro's product. If you're curious to see what many years of use does to one, you can see a comparison of the one removed from the 1959 engine and a new one:
http://i1154.photobucket.com/albums/p535/JonGoodman/rearmainendtoend2.jpg
and another view:
http://i1154.photobucket.com/albums/p535/JonGoodman/rearmainendtoend.jpg
and another:
http://i1154.photobucket.com/albums/p535/JonGoodman/rearmainsidebyside.jpg
and two rear main caps...1956 and 1959:
http://i1154.photobucket.com/albums/p535/JonGoodman/apairofendcaps.jpg
and note in the 1959 cap...raised notches have been created about 1 inch apart...I presume to keep the seal from rotating:
http://i1154.photobucket.com/albums/p535/JonGoodman/closerinspection.jpg

Also as an fyi, I got up in our attic this morning and found some old rope seals. One pack dates to around 1975~1976. One has no date but I think it dates to around 1985. The oldest one looks like it has dried out. The other one is more flexible. I'd be happy to trade them for one of the Best Gasket rubber seals (for comparison).
Jon

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end