jmoore - I'm no expert when it comes to GMC Master Parts Books, but a search through
GMPartswiki for GMC Steering Gears, shows the below details from a 1963 Parts Book covering the years 1955 (Y), 1956 (X) and 1957 (T) for your Model 150 (15 as shown in the book).
There does appear to be a specific "w/4 Speed Trans" gear box for a 150 for 1956 (X) and 1955 (Y) - GM# 5668106. Then there is another 4 speed specific gear box for 1957 (T) - GM# 5671458. Also listed is steering gear for the 1956 (X) and 1957 (T) trucks with power steering - GM# 5684268. Separate from those 4 speed specified gear boxes, there are 3 different part numbers for "STD CAB" (Standard Cab?). One for each of the model years 1955, 1956 and 1957. My best guess is those represent the standard 3 speed trucks, but again that is only a guess on my part.
Perhaps some of our GMC guru's can shed more light on your question???
