Imperial German Regimental Markings
 

Are they applied at the factory or in the field?

NOT at the factory. In the "field" at the regimental or company level.

I thing Kreighoff is the exception

Gunbugs is correct. Regimental markings were applied at unit level by the unit armorer. There were specific regulations on the manner in which they were to be applied but the rules were not always followed correctly. That's why there are variations in where and how they were stamped. I have two Artillery Lugers assigned to the same regiment. One is marked on the backstrap and one on the fore-strap. I suspect that they were marked by different individuals.

Those aren't unit markings.