That 48 head has the pressed in nozzles to direct coolant to the backside of the valve seats. I would imagine they were prone to clogging, hence their discontinuation shortly after 1948. Just my guess, since it appears that GM killed everyone with any real knowledge about these trucks, upon termination or retirement (another one of my guesses smile ).
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission