I asked. The private owners shrugged and said that their father liked to service it a lot.... So on opening we found this encyclopedia: The watch itself :
Note the distance between the subdials - much greater than on a standard Speedmaster dial. Indicating this execution is on a whole new dial plate, with smaller sub dials. Indeed I have seen Racing dials with damage to the Subdials due to incorrect hands fitted (too long). Unlike the Non-Pro version that has the same dial plate as the black dial. To me this makes the Professional Black Racing dial even more interesting.
I'm surprised that it was able to survive until now without having parts replaced, especially considering that the original owner liked to have the watched serviced a lot. Excellent find!
I would expect that case to be polished to a nub with that many services- that watchmaker should be commended for the careful handling.