I now find out that it was rechambered for a .410 x 2 1/2" case, to make it legally a shotgun (with no intention of it being used as a shotgun) which in the 1960s/70s was not uncommon in the UK at that time to prevent it being classed as a firearm for which a certificate was required GRRRRR

I am chasing a guy who has made cases for me in the past to see if he can make an insert
I have also talked to another guy who, as Lancaster says is able to TIG weld the extractors to fit the correct cartridge

Many thanks for your input

TH