There are two PW 773 variations. Basically delineated as the following:
The 1950 Production run has a fancier valve gear assembly and came with the premium 2426W tender.
The Fancy Valve Gear:
The 1964 production lacks the fancier valve gear and came with the PENNSYLVANIA 736W or NEW YORK CENTRAL 773W tender.
Lacking the valve gear:
The parts list shows 773-55 as the valve gear support for both the 1950 and 1964 versions... can you elaborate?