Hi Fleet16b:
In response to “I have an actual mould that was used for making these stop watch holders”.
Wow! What a desirable item to have; a great find. Could you post a few pictures? Particularly of the (reverse) impression of the lettering. I think I have see a few different examples of the raised format so perhaps they were made under contract by a few companies?
Maybe, rather than you finding a replacement formula, you should try to find a company that makes traditional real rubber items and have them do a limited run with your mold (put me down for one too). I believe someone in Britain did this a few decades ago in both black and white versions and those particular high quality replicas were still selling for a hundred to 150 quid or so (or a gazillion Canadian Dollars) recently. I guess it depends on how authentic one wishes to be. Certainly silicon and the modern urethane’s would do the trick, look great and last much longer but they’re just not, … “real”.
Ya, 3D printing. I guess I have to get with the program. It’s hard for an old phart.

Thanks,
Wayne