Yup, easy. Clamp all three in V blocks side by side. Add a fourth scope of known power (20X if possible) in a fourth set of V blocks next to the first three.
Set up a large target made up of 1" grids blocks at 100 yards. Look through all four. Any difference as large as 12X should jump out at you. On the assumption that all 3 Lymans started with the same FOV, some simple math and grid sqare counting should allow you to calculate the power of the modified scope.
These were good scopes in their day, almost as good as Unertls BUT no match for a current fixed power Nightforce of today.
You might want to talk with TK Lee optics as they do excellent work,
I gotta get out more, didnt know Nightforce made a Fixed Power Riflescope.
Gunner
