Howdy, Mark!
If you go to the top of our page you will find the "Link to Articles & USFA Catalogs" section. Having just reviewed both listed catalogs I note that the only Bisley model listed is their "Holland & Holland" model. No mention of it being available in a flat-top version.
That being said, USFA was producing a flat-top version of their SAA so, in theory, you probably could have special ordered a flat-top Bisley but it would have been by special order only. The flat-top Bisley was, apparently, never a production item. The chances of finding one would be extremely rare and dependent on the very few, if any, special order Bisley models.
I'm just guessing here,
CJF