PS-Two oddities on this gun's barrel. One I figured out, but one eludes me?
There's a small dovetail in front of the front globe sight dovetail. That one turns out to be for a scale indicator so you can watch the globe sight movement, and see how far you've moved it.

[Linked Image from i.imgur.com]

The other is a second dovetail over the chamber area, which I suspected was for a spirit level. But it's only a 1/8" dovetail, and seems very narrow, even for a spirit level?

[Linked Image from i.imgur.com]

I'm still puzzled, and have no clue for sure what it's for?