Janne;
I noticed that your questions weren't all answered, I'll do my best, though my knowledge, like me is kinda dated.... LOL
Cal 7.65 mm was used mostly for commercial and 1900-19?? contracts. 9mm was considered by most countries as the military round and most guns built for military use or export were in 9mm, at least after the start of WWI (some exceptions to this of course, just like all things Luger).
The letter with the serial number is because the used a 4 number series with military Lugers that repeated itself. like 1 to 9999. Then they started over with 1a to 9999a and in this case 1b to 9999b with yours being the 887th luger in the 3d series or the 20885 th Luger that year. Commercial numbers for DWM ran consecutively from 1 into 5 digits.
I would assume that the deduction of an artillery background is based on the notch you see at the very front of your chamber, just behind the barrel. That was needed for the fitting of an artillery rear sight. A different hypothesis could be that the receiver is from a WWI Erfurt Luger that usually had those notches even when not used on artillery Lugers. In any case the gun was re-barreled to 7.65 for some reason.
Keep digging. It is a lot of fun.
Gary
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