Questions for a new Luger owner

[phil.luger]

Hello all, I received this Luger from my dad a few months ago and have a few questions.

It is 9mm

I know the drop down lever is wrong, a gunsmith said it may be off a Stoeger of the same era...or re-made.

Also, I don't have a pic, but I would really like to know the meaning of the numbers on the metal frame between the grip, below the trigger. Mine shows: 1./N.3.17.

Any insight to my gun, history, etc...much appreciated! Thanks, Phil