The three opening bolt face, is the one that had the 3-piece firing pin that needed special ammunition only issued to the UK Police and jails.

With the 3-piece pin, the gun would not discharge normal shotshells as the two outside projections would stop the pin before it hit the primer by hitting the shell's head beside the primer. Issued buckshot ammo for these guns had a groove around the primer for those projections. This might have been a rather short sighted technicality if one of them has a screwdriver and a file along with a modicum of imagination.

YES - tig some material for the extension and file/grind it to shape for a 16 bore, or smaller casing.

I've had to do that to several guns and it results in a perfect fit.

I think the pins might be just fine for BP level to 1,800fps loads with 600gr. bullets? Any comments - welcomed?

The left one is too long, of course. Much depends on the headspace, but that is about double too long by the looks of it - don't know why. Look at the other one's projection.

I suspect my Martini has that long a pin as well. I'll have to measure it with a rod.

If the pin is like yours, a .450 Alaskan for 1,500fps, 500gr. loads might be in order, as I already have the reamer (2chambers cut), as well, I have a .458 'American' piloted reamer that has cut only 2 chambers as well.