Found on Reddit:
H&K MP5 SD tool room prototype: The QD thing is interesting. Looks like a toothed gear system to an interrupted thread
H&K MP64 (MP5) Serial number 3:
H&K MP5K Serial Number 1:
H&K MP5 SD tool room prototype: The QD thing is interesting. Looks like a toothed gear system to an interrupted thread
H&K MP64 (MP5) Serial number 3:
H&K MP5K Serial Number 1: