The Quadrajunks were jetted differently for a certain application--i.e., one for a 455 Olds 442 would be richer than one for a 327/250--the secondaries would be calibrated to open at a different vacuum, etc... Actually, the Junks used metering rods instead of jets, unlike a Holley, which was used pretty much only on performance engines. As to how GM determined their usage I have no idea. But it makes sense to me that mass-production would demand they be as generic as possible, as you allude to. Somebody on here will know. Hope this helps in some way and good luck! I'm now as curious as you about how the I.D. tag was determined.