The grip frames are interchangeable between old and new models. However, I believe the new model "medium" frame guns require the "tabs" be filed off though.
All bisleys are new models. they were not available as an Old Model. It is possible to fit a bisley grip frame to an old model. The trick is the hammer though. There are no old model bisley hammers......the trick is to have a smith replace the standard spur with a bisley spur.
I also believe David Clements produces batches of bisley hammers for old model lockwork from time to time. Power Custom MIGHT also have something that will work. i know they had some bisley stuff made up to use with their #5 grip frame on old models.
that's probably a lot more than you wanted to know, so just sift through the poop
And welcome to the forum