Yeah I think it was his blood group and ID number... but this is long ago... I think I also saw the heart shaped insignia of his squadron - it was all done in the same blackish ink... He would have been 60-ish or very early 60's, so taking taking 40 years or so from ~1982 would have made him around 20 to early 20's, when he was flying...
I want a suitable plane of my own choosing, and a train tunnel, and some time and resources, to practice, and I will fly it through a train tunnel.
If you have the desire to express this opinion, I'd be willing to assume that you also have the money to fund the venture, as well as the expertise to go with it.
If not.....
I saw his tattoo's for perhaps 30 seconds. He pulled down his overalls - the top half, and rolled up the sleeve to his short and showed me them. Now at the time, I was not having to expect to recall every detail precisely some 40 years later... So my gut feeling is that in terms of practicality and legibility, that the tattooist would be using a "military" type font, meaning no fancy flourishing - and as easy to read as possible - as quickly as possible - even when skin damage has occurred. Not sure of the exact font type, but legibility and ease of reading, take precedence, kind of like the US military font. I am not saying the Germans used "A Font" - and the tattooist had a heap of people too tattoo, and they have their own style, so it would have had to have been neat hand drawn lettering, in a very legible font style....
https://www.tlai.com/med_des/amusaf.html