Thanks for your replies.

It's something extrange in these marks. It looked like there was a zero under the eight, but the chamber doesn't been sleeved down to 28 ga from 20 ga. See the picture below.

[Linked Image from i.ibb.co]

28 bore diameter: .550
24 bore diameter: .579

So, what do you think? May be, a bad day of the person who recorded the marks?

28 ga, hammerguns and all shotguns and rifles made by hands.
Waidmannsheil 🌿📯