Tinker,
Sarg had his Tolley 12 bore posted but I can't find it now, maybe posted 2 or 3 years old. It is a beautiful gun.

Sarg
I checked the twist on my Holland 12 bore and it has a similar twist 1/60 or a tad slower. Records show it was to be used with ball or conical. The sites are a single fixed 50yd 6 or 7 dram 13lb. So you need bore and groove measurements. What is your 12 doing "out of the country"?