OMG Gudabeg ... what a depressing roller coaster ride of joy and pain. .... sorry for your troubles.
The good news is I think that your gun is a genuine dutch Luger ... the fact the 'plate' has been removed at some point does not necessarily mean it was done with evil intent.
I can see some novice owner not understanding the plates importance and removing it for purely cosmetic reasons ... mores the pity.

and then re-attaching it/one for the sale.
What would I do ???
1st
Quote:
Originally Posted by mrerick
adhesive built up on the plate may be an issue if not removed before you try and adhere it again.
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Not wanting to apply any heat I would forgo the use of soldering or brazing at this time.
In my youth I used to use a wonderful product from Goodyear rubber called 'Pliobond' to successfully bond metals .
It is a simple to use solvent based contact cement that bonds most materials but specifically metals.
You simply coat BOTH sides and let dry to the touch BEFORE bonding ... 2 coats is the best, let the 1st coat dry before adding the 2nd coat ... guaranteed success.
Once both sides are dry you then press them together ... but get the 'line up' right because removing them after contact is difficult and requires chemical assistance. ... that's why its called a contact cement.
It is readily available in Canada but there are restrictions in the USA.
"Pliobond 20 cannot be sold to or used in certain states due to the EPA regulations regarding VOC content. Pliobond 25LV can be sold and used in all states".
https://www.handlaidtrack.com/assets...-datasheet.pdf
Good luck whatever you do ... its an awesome nice pistol.