if it have a 8x57 IR or IRS chamber it will be nitro proof gun and safe for 2,5" shot loads. please post a photo of the proof marks. collath dont make saturday nigth special guns. it was a best gun maker and in my opinion on a level with th best british firms.
the collath No.4 and No.3 are very similar and I dont find my data list now. if the 16 bore fit than you are rigth with it. the teschner- collath shotgun cartridge family is full with mystery's.
please make pics of all mark's.