I agree with Doubs DarkBiker98.
The front strap marking is a police property mark. S.D. IV 1251
The Schutzpolizei (police), Düsseldorf, Fourth precinct, pistol number 1251.
The sear bar seems to me to be 'fiddled' with.
The exact date of post-WW1 commercial manufacture is best left to others to explain.
The Walther safety (magazine safety) was intended to block the trigger when the magazine is removed.
Thanks for sharing your gun ... I really like it ... but then its a rare luger that I don't like.