The Pro P8 lineup didn't have a 550W model as far as i can tell pasty. The straight power lineup did, but then it wouldn't match the description OP has provided, it definitely won't be the P11 as i incorrectly assumed earlier though(that'll teach me for quick-googling a description), given the age.
Could possibly be BQT P7-PRO-550W i guess, bit harder to find info on but coincides with the age. Assuming it is that PSU an upgrade really wouldn't go a miss at this point.