Andreas,
As written by Lorant and also I agree
IV./JG 300 was formed from I./JG 76 beginning October 1944. Planes were flown to IV./JG 300 between 4-10 october...so from 4th I think it may be reported as IV./JG 300 loss and also mismatched to JG 76.
However if there is loss report to IV./JG 300 before this date, I believe that order to rename the unit came earlier and someone already applied it before it was valid?
My opinion
regards