Hi,
The trigger plate is the plate on the left side of the receiver (removable) that has a lever connecting the trigger to the sear and it's plunger. It's exterior is plainly visible in the second new picture you posted. You should find the number "3" stamped inside it.
It looks like your pistol is missing the small take down lever retention spring. Don't shoot it without installing one. As you Luger looks collectable, it's not a good idea to shoot it anyway!
You can find disassembly instructions on this site.
Marc
__________________
 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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