Welcome to the forum Kato. The original truck engine in your '55 would have the engine no. ending in F55X, so I'm not sure what the extra "2" is for, possibly a remanufactured engine?
The pushbutton starter was only available for trucks equipped with the Hydramatic trans so I'm thinking someone swapped in a SM420 4-speed somewhere along the line.
The rear bumper sounds like an aftermarket item, there were lots of them available over the years. The Barden bumpers were commonly seen on pickups over the years.
Hope this sheds a little light on the situation. Post some pictures when you can - we love pictures!!!
