It is a 55 degree thread as noted by Luger doc, but "specs" are a combination of metric and English;
18mm x 20 TPI.
I have a little different opinion than GT on 55 vs 60 degree compatibility; I've cut many 60 degree threads and threaded them into original 55 degree receivers.
Each receiver is different and "real" luger barrels from the day will not necessarily interchange into "real" luger receivers.
I find that when I grind the angle on my own thread cutting bit, I'm not good enough to tell 55 from 60 degrees!
Cutting the thread just a bit deeper will allow the barrel to screw into the receiver; the draw at the flange is what keeps the barrel nice an tight anyway. JMHO.