A quick search shows the early 3.5" 12 ga and the 10 ga were on the same frames. Sometime in the 90's the 3" BPS was redesigned to handle 3.5" shells. There has been at least one other design change in the 12 ga receiver since then but I am unsure if it had an impact on dimensions.
Since the 3.5" 12 ga now shares the same receiver size as the 3" model, I would look at mounts specifying 3.5" as it would be a reasonable guess that the trigger assembly pins spacing and receiver width would be similar with the 10 ga. I would call the scope mount manufacturer and ask to talk to someone in engineering or production and ask them what the difference between the 3" and 3.5" models. That would give a person a pretty big clue which/if a particular model of base will work. Often direct contact with a person will provide information quickly and easier than trying to find obscure information on the web.
For whatever reason I have 1994 in my head for changes on the BPS. My bps is late 80's. I emailed a company, they told me they thought the 10 gauge receiver was a little more beefier.
