DD:
Well, that's interesting....

..unless your Martini actions are Martini Enfield, not Martini Henrys.

I have 2 MHs, an arty carbine and a 3-band rifle, and one GP, and have measured one other GP. All were an inch across the breech-block, give or take a few thou.

Perhaps there is some variation between makes (lets discount the Khyber Pass example, at least!), but I don't believe the GP breech to be consistently or deliberately larger than the MH.

I have the Martini Henry Notebook at home, will check for possible clues this evening.

If this action takes the full force of discharge on the hinge-pin at the rear of the breech-block, then the GP action must be weaker than the MH action because of the removal of metal behind the GP hinge pin to lighten it for shotgun use. If you are building a big-bore rifle on this action, that could be an undesirable feature. Just ask Graham Greener what he thinks of the practice of building big-bore rifles on the GP action, then stand back!

Your point about the safety-catch is a good one though, and may be the critical factor in some jurisdictions for legal reasons.