Mike Connealy , May 12, 2008; 08:11 a.m.
It is an uncommon camera, so you can count yourself lucky if you have one, but it is hard to find information about them on the web. There are a number of medium format folders which feature dual formats, and I think most use clip-in masks. My Dolly Supersport has masks for both 6x6 and 6x4.5; both masks in this case have their own rollers. It may be possible to make your own mask using some thin sheet brass or similar material. The important things would be to make sure the insert has nothing that would scratch the film as it travels across it, and to keep the surface at the correct film plane distance.
Have you looked at Brian Wallen's pages on the Regent? Unfortunately, his photos don't illustrate the 6x4.5 mask, but he does have some good notes on the restoration of the camera. Perhaps he would also be able to provide you with some information about the mask.