Hi Graham, it's the standard 55mm from the M42 to M48 adapter.
That though is a custom modification because standard it comes only with a M42 thread, with the provided extension tube (which I still have and will include),
the calc is as follows: 83mm from the reducer shoulder thread, where the provided extension is 28mm.
See attached diagram.
Also attached a test shot with the full frame camera to show the extreme corners
The full docs are here:
ASKAR FMA180 manual