Or at least some of them, I’m not sure they are all built the same.
When my Heligon 42/0.75 arrived, a few years ago, it looked like in the first picture below. On this build, the optic block is in one piece, screwed in on an internal 48mm thread with a photograph friendly 0.75 pitch . Adapting it was easy: unscrew the optic block, screw together a K2 and a 48-52mm step ring, screw it on the internal thread, that’s it. For other mounts, there's plenty of 52MM reversing ring to use instead of the K2.