Great find.
It is safe. The rattle that you are referring to is the dust cover that is attached via the 2 split pins that are visible above and below the buffer. The buffer is complete and aside from the split pin issue, I don't see anything wrong with the stock. You might talk to HKPRO member gunplumber and see if he has devised a way to correct the issue with the split pins; I know he is aware that they can become loose with time. I have an A3 in similar condition that I am considering disassembling to see if I can remedy the pins.
Best,
TC