Fede - you mention that the shoulder diameter is wider on the 7.65 Leuchtspur training cartridge. Do you mean by design? I measured the following three cartridges from my collection the best I could, not knowing the exact point of reference for the measurement you were talking about, and got the following results:
7.65 Lsp D 4 T 74
Just below shoulder: .3765" (9.57 mm)
Just above shoulder: .3295" (8.37 mm)
7.65 Ordinary Ball D 4 T 74 (Yes, just happened to have the same quarter and year, also with blue PA.
Just below shoulder: .374.5 - .376" (9.51 mm at smaller diameter)
Just above shoulder: .330" (8.38 mm)
7.65 Ordinary Ball D D 11 T 57
Just below shoulder: .3755" (9.54 mm)
Just above shoulder: .329" (8.36 mm)
I would say that all three of these rounds would interchange in a normal specification chamber. The just seem to be in the same range of specification. Perhaps I am wrong? It is not easy to measure right at the bottom of an incline, and not so easy at the neck of these rounds, due to very deep, longitudinal neck crimps. If the caliper gets into one of the crimps, and they are big enough for that, it changes the measurement enough to be significant.