Hy spanners, the transmission you are now describing sounds like a Dual Range Hydramatic, which did not have a "park" position as such, but as you said when you placed the shifter in "R" with the engine shut off you engaged a park prawl. Unfortunately 1955-56 was the time of changes in the transmissions in cars, somewhere in 1956 most cars started being equipped with the "Dual Coupling Hydramatic". In Canada (where I'm from), our trucks never changed from using the "Dual Range Hydramatic", they used it right up to 1962. In my mind a "Dual Range Hydramatic" is a far superior transmission to the "Dual Coupling Hydramatic". It would help to know if this running gear came from a car or a truck (American GMCs used Pontiac V8s from the 1955 2nd or "B" series in their V8 equipped trucks up to the 500 series), so a little detective work on your part is required to verify which transmission is in your truck, hope that helps.
