If you examine enough Lugers you will see that the hidden surfaces are stamped with all manner of Roman capital letters and other, more cryptic marks. These are Workers' marks, probably denoting completion of a part or examination by a factory inspector (rather than an Army inspector). Görtz & Bryans ("German Small Arms Markings", p.114) quotes the relevent Army instruction which permits these stamps, all letters of the alphabet except for A, which indicated a rejected part. Numerals were also not permitted. With the exception of A, the specific meanings are unknown.
A Luger collector named Charles Sorrentino was doing a survey of these marks in AutoMag a few years back, but no conclusions are so far forthcoming.