1917 erfurt
 

I recently saw a 1917 erfurt with a slide that was different than other lugers I have seen. It had a thin piece of metal on the left side. Is this what is called a j-block? What is it's purpose?

IF the think piece of metal was riveted to the hood that covers the trigger bar and then extends over the side plate, it's a sear safety and indicates modification for use by police.

The purpose is to drop a pin into a hole in the trigger bar when the side plate is removed and prevent accidental firing by someone depressing the trigger bar and releasing the firing pin onto a live round in the chamber. The Luger should NEVER be disassembled with a chambered round but apparently it was enough of a problem during it's service in Germany with police to warrant special consideration.

Thanks, I might try to buy it, but I don't know if it's possible right now. It is a private owner. What would be a good price to offer him?

"What would be a good price to offer him?"

There's no way to tell based on what we know about the pistol. It depends greatly upon original finish remaining, matching serial numbers, bore condition etc. Imperial pistols modified for police use actually seem to command less money than one left in it's original state. Assuming a 90% or more original finish, VG or better bore, and all matching numbers (magazine may not match and that's OK) you might offer from $650 - $750 depending upon how much above 90% the original finish is and how much straw coloring remains on the small parts. If finish is below 90% then I'd offer maybe $500 - $600. Just my opinion but I think I'm ball-park.