First your gun was made likely by Crescent (later ones by Iver Johnson) and is close to 100 years old.
12mm was a shot round from long ago, but the gun could also shoot .410 shells- WHICH AT THAT TIME WERE POSSIBLY 2 INCH SHELLS. 2 and a half came later, then 3 inch. The only way to tell for sure, lacking markings, is take it to a competent gunsmith who can check with a chamber gauge.
They were sold by the Hibbard Spencer Bartlett Company of Chicago, Il, a large wholesale sporting goods dealer. If you look real close it may be a model WN.
Choke- with those .410s, all or nothing. That is a full choke. Smith can check the choke with a choke gauge to be sure.
Slugs are not your problem- a slug will go thru any choke EXCEPT an Extra Full Turkey, or a an old Mossberg "dial a duck" adjustable choke (C-Lect Choke) Your problem is getting the gun checked to determine the correct ammo.
PS- yeah, I love me some 410 topbreaks. My first gun was a Harrington & Richardson single shot 410. Gift from my Grandad- on the day I was born.