Your Questions followed by some answers/opinions:
What does S E S mean on the right side receiver?
Inspectors initials applied at various points in mfg. and after proofing. This is covered in the stickys.
There are no import marks. Can I assume this was a bring-back?
yes
There is a J engraved in the right side grip. I assume this was done after the pistol left the service?
yes
What would be the approximate value of this pistol?
Check online sales
Value of holster?
Not much, looks to be aftermarket or ?? not German WWI military
Would you consider this a collector or shooter? I do plan to shoot it. Do I need to take it apart for this question to be answered?
You need to take it apart to clean it and inspect the workings, and to lubricate it.
Collector or shooter-
yes and yes- if you want to.
If you want to share what you paid, we can say good or bad on the deal-
many of us don't do "valuations", it makes no one happy and one is never correct-
There are just too many variables and every one has an opinion.
