Welcome to the board - so far I've not encountered a back clamp buttonholer. I'd love to see if there were any buttonholers made for a machine like a Wheeler Wilson. I have very early buttonhollers that are bed mount, side mount and round bar mount - none of which work with the back clamp. I do have early sewing books that describe a method to make a machine buttonhole, but it's more like FMQ around the opening.
One would think that Singer had something available for the 66-1 since it was made for about 15 years. It and the D9 (that I've read the 66 was modeled after ) are oddities for Singer in that they are not side clamp.