Take a close up picture with a high resolution digital camera, not an I-potato (phone). Cell phone cameras suck.
It's looking like the "proof mark" may be very similar to the one I've posted below, which is a Nazi-era Hza proof of a depot-level rework. Your photo is actually pretty decent, but for positive ID, you need to get a good, sharp, close-up image. The higher the resolution, the better, so that one can 'zoom in', and really get a good look. My photo below was taken with a 14.1 megapixel digital camera. My photos are only 'OK', at best, but they serve the purpose!
Here is an example of what you need to do:
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