In this place and time, it was probably a fighter-bomber group from the 9th AF that attacked your town. If the aircraft were correctly identified as P-51s, they were probably from the 354th FG.
Other P-51 units flying that day will probably not carry bombs, while RAF units, especially Mustang ones, will probably not fly a fighter-bomber mission in this area.
So my first guess will be to try to find a war diary for the 354th FG.
On a side note, I find it rather strange that a German says that his city was liberated by the Americans in 1945.
