During the New Design series of GMC trucks (approximately 1947-1955), the 54-55 body style was called Z series by GMC. GMC did not seem to make year-references (at least in vehicle/chassis serial numbers) during the years below.

A = 1939-'40
C = 1941-'46
E = 1946
F = 1947-'50
Z = 1954-'55 (Dec 53 through 15 Mar 55)
Y = 1955 (after 15 Mar 55)
X = 1956
T = 1957
S = 1958-'59
N = 1960

Rob English has some helpful information on these GMC series designations. Rob has a large collection of GMC documents - you might want to contact him about 1958 information.

http://www.oldgmctrucks.com/page0010.html
http://www.oldgmctrucks.com/page0290.html

The parts suppliers might use any type of year-designation terminology.

Tim


Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.