I have a Busch C also and it's the type with the small carbon like Lens board. I did make an extra one but the screw holes and board fit are difficult to get right. It the only problem with an otherwise fine camera. I wish I had the model with the removable lens board.
I've never seen any for sale, they would have to be made and to change them requires a screwdriver and some in the field. The other problem is the Kalart is adjusted for the 101 lens and it isn't a cam setup.