I bought this watch on an auction. Just wondering if someone could help me authenticate it. The back and the crown aren’t market or anything so I got a bit concerned.

 

Looks OK, needs a service and a new crystal.

A under valued brand in the vintage world.
It will make a good daily wearer once serviced

Do you have a picture of inside the caseback.

I have two a bit later than yours

 

I think it's a nice watch and the harsh lighting doesn't do it any justice.
It's imaginable for the case back to be blank. The crown - I believe - should be branded, though.

It'd come down to how much you paid for said watch and to how much you like it.

 

I paid around 160$ for it. I haven’t opened the back myself as I don’t have the tools to do so. Therefore I haven’t checked the inside of the case. So you are saying that the crown most likely is replaced with an off brand?

 

Just a replacement crown done at some point in its life. No biggie. They wear out.

 

A Schild AS 1687 made for GP.

 

Get it cleaned up, wear it and enjoy it!
They are an under appreciated brand from back then