On my 53 the PO took a key and filed off the ridges. It is smooth across the top and turns the tumbler. You may want to try this if you have an old key.
I plan to take the tumbler out of my 53 and have it keyed like my 55 2nd. I will do the same with the door lock. Probably put a door lock on the drivers door, too. I did that on the 55.
Just a suggestion.
