John,
DoubleD is the Martini Guru, if you can get him to chime in it will be more than worth your while & everyone else as well!.
He is a treasure trove of reloading data, cartridge information & frankly everything Martini!
I really admire your piece there mate, it certainly is a looker; shame about the recoil pad, but they certainly do generate a fair old thump so its a vital necessity over the old steel butt plate.
I was about to get my MH running when all the virus crap landed on our doorstep & so am in limbo once again.
My hope is to get out soon with a mate to a property & trial a few different calibers, especially my old martini.
Will let you know how I go.