Too be honest I only know that it was made in England and has an Axminster badge. Someone on another thread identified it as a super precision but it may not be.
There is a raised spigot on the back but it’s quite small compared to many.
It could be that a previous owner had an after market Blank Back Plate machined or tapped with the 3/4" thread to fit the lathe and it was not machined true.