Are there any marks over the 'V' or 'JD' that the camera didn't pick up?
These and the numbers are not in themselves proof marks. Most proof marks are a letter and some sort of symbol, be it a crown, eagle, or other symbol.
Shotguns of this era, depending upon country of origin, will have the proof standard stamped on the barrel(s) in grains of powder/grains of shot, along with the proof mark.
Keep looking. There is something there somewhere. I would start with a magnifying glass as some of these can be pretty small.
Can you get a better shot of the image in post #3? This is probably your proof mark. I might be able to at least identify the country of origin if you can get a good photo of it posted.