A very interesting example of an early Parker in any event. Never say never and hope you can verify the gauge

Nicholas, that looks like a nice early back action lifter. My “guess” is that it is chambered for the Parker 12B cartridge, which was rather short lived. It is about the same as a 14 gauge paper shell if I am remembering correctly. I am showing 2 different guns below. The first is serial# 97 that is chambered in 12B. You can see the inside measurement of the bores.

I did put a 16 gauge shell in and it is loose but fits. It almost will slide past the extractor. If you have access to 16 gauge shells you can test this. I could see someone who was not as addicted to Parkers as some of us, quickly putting a 16 gauge shell in and assuming it was a 16 gauge. This would be very dangerous to shoot.

I also posted some pictures of an early 16 gauge lifter serial# 16247 measuring it the same way and you can see it is slightly smaller.

The 12B shell is obsolete and you likely won’t find any 14 gauge paper shells either. I did have Rocky Mountain Cartridge make 50 - 12B shells copied from an original I sent them.

I hope this helps. Let us know what you find out.

*Quick Edit...I know I need to get some better measuring tools, it's long overdue!

I bet Jay is right. As you can see, he knows hammer guns