Keep in mind that there is another angle at play here that may be warping the interpretation here in your main reference...
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The slightly toed-in angle of the scope/rail/mount relative to the centreline of the barrel (probably due to the taper of the Mauser lower) would make the scope appear to be sloped downward when viewed from the side/bottom angle like your main Stembridge reference. So while it's possible there is a downward slope to the scope when viewed from the side, this other angle will also be a factor to contend with when trying to nail down the true geometry of this thing!
Yes,you are right, it's hard to nail these angles, especially from a single picture. Also parts can be loose or re-assembled and things to change from picture to picture.
I mean the angle in this mount for example is pretty obvious and easy to replicate :
but for other it is really hard to tell what exactly is going on.
And I'm not really trying to nail everything it in the build - just to the point it stops bugging me
and of course to the point where I can realistically make it with my tools and skills. For example the holes for the mount are pre-drilled, if I think they are off by little I can't change it anyway.
But nevertheless I'm curious to figure as much of these little details as possible and revisit them again if someone notices something that I've missed or a new BTS picture surfaces.
For the angle you are talking about there are just too many variables - As far as I can see the mount is fairly straight and uniformly thick and the spacers below it are also the same, but even a small difference there will be visible in the angle of the scope.
Then whatever angle happens naturally because of the shape of the mauser body should be repeated in our builds for the same reason as the steel mauser is a very good replica of the real ones.
On top of that there are a lot of parts that are probably not perfectly aligned to each other and can skew our perception how big is the angle and which part is causing it.
I think the rail is very well aligned with the mount, but from there are so many things - the scope ring can be tightened at a small angle, the scope could be tightened off center in the scope ring clamp, the front and the back part of the scope can be assembled at an angle, the scope tip is little on the side for sure, the flash hider could have some play around the barrel as well.
I can see the angle that you marked with the red lines in this and many other captures, but in others it looks pretty much parallel with the barrel and in some it even looks like the scope and the flash hider are angled away from each other:
Very hard to tell also for the other angle that chubsANDdoggers brought up - was it properly aligned during the filming?It's impossible to tell Here are the best captures I have: