I'm willing to believe it's a sight, but it raises a lot of interesting questions for me.
If it is a linear scale with vernier, and the angle is relatively small (minutes of arc) I can see that it might serve an angular purpose since a small range of angle approximates a linear scale better.
But that would mean that the entire range of the sight would be only plus or minus one degree, which seems like a very small angular range for a gun sight.
If it was degrees instead of minutes, then it really would be an inaccurate measurement if you impose a linear scale on an angular range of 120 degrees total (+60 to - 60)
If it is like the adjustment on a peep sight adjusmtent to bring it into conformance, a small (minute or second) amount of adjustment would make sense because it is in relation to a second sight point on the barrel, but isn't that usually "clicks"? in other words, relatively arbtrary. Why then would it be necessary to have an inconvenient numbering system like 60 instead of 50 unless it was important to relate it to an angular numbering system? The 30 mark and 60 range makes me think it is angular, but the range seems too be either too small, or too linear for sighting.
I'm sure there is an explanation, just curious how it works and why (as usual!).