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Just clean up the lenses and replace the post and cross hair.

Here is an original Flanders flap.
I have had it for about 50 years and there are two or three others that I cannot find,,but they are somewhere here!!!!

They are probably using the horizon sighting method when shooting at the train.
The picture gets blurry when I try to expand it and I cannot see the rear sight close enough.
Same system used with...