I was wrong in stating that the 55.2-60 GMC Master Parts Book only lists one part number for the Zenith adjustable main jet. That book does not list the adjustable jet at all.
The 1939-53 GMC Master Parts Book DOES lists just one part number for the Zenith adjustable main jet. 2233002. According to that book (see first picture below) it was optionally used on 100 through 350 series GMC trucks in 1939-50.
The second picture shows this part number (2233002) in a "GMC Engineered Accessories" catalog from 1950. There is a very interesting (to me anyway) statement in this document that says "and can furnish normal mixture when the convenient adjustment is turned all the way out". To my feeble brain that means that the original high speed jet is still in place while the adjustable jet is installed. How else would the "normal" mixture occur when the adjustable jet was full rich?
To support my opinion I've included the third and fourth pictures which are taken from a Zenith service manual dated 2-1-61. Item 53 is either a cap or the adjustable jet. Note that there are two parts listed for the adjustable jet, one "used with main jet" and one "when no main jet is used.
I'd like to get CarbKing's opinion on my comment about leaving the original jet in place when the adjustable jet is installed, even if it is that I am full of $#it.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
