Ed, you are talking about the newer census code, later revised to equipment configuration code. The correlation between the old code and the new configuration code isn't that direct.
I just checked my manual from 1960 and can confirm that the old census code for the 1/4 ton ambulance (both M38 and M38A1) is 2236. There are also 2238 (121106) and 2239 (121107) which have a different stock code for the kit, and the 2239 is with an interim radio adapter kit. The ECC for the old census 2236 amb is 121103.

Census Codes vs Equipment Codes
 

Ah, so not only is tracking the AoS markings a challenge but at some point in the early 1960s there was a change from the Census Code to the Equipment Configuration Code for military vehicles. It is good to see that 2236 does relate to the Ambulance and yes, I misread my ECC list as ECC 121103 is for the M38CDN Ambulance.