You shouldn't need to disassemble your Luger to see a breechblock proof, if one is there. It will be stamped on the part which humps up above the receiver, I've enhanced it in the photo below.
The 1920 is almost certainly a property stamp, not a date--judging by the manner in which it is stamped, and the serial number not being characteristic of 1920 production.
There is no sure way to know who did the rework , or when. Considering that it is 9mm and has no Police safeties, it was probably reworked for army use.
Your original question runs along lines that I myself have some questions about, which is why I followed up--I hoped for information which will help me frame my own questions.
--Dwight