I don't usually pay too much attention to model numbers, but I have a 555 55. The 555 indicates long shaft, electric start, magneto ignition. No alternator, so, periodically, you will need to charge the battery.
I just bought a very well maintained 1976 Chrysler Commando 154 and I assumed they stuck this older 55 on it to sell it. Not an issue as all I wanted was the boat.