Based on the history available to me so far, distinctly Hilgen amps were made only from 1964 through 1967. I had run into a string of 1965 and 1966 models that had chassis codes that ended appropriately in 5 or 6. Hence I tentatively concluded that the last digit was the year of manufacture. Since then I have found chassis codes with final digits that cannot possibly be the build date, and others where NONE of the digits could be year codes.
The letter preceding the number is proving no more consistent. Thus far I have seen chassis codes beginning with the letters B, C, D, G, and K. Unless there is some startling new information out there, there are already more letters than there are possible years of manufacture. I suppose the letters could indicate the month, but it seems odd that Hilgen would keep track of the month but not the year. The letters definitely do not signify a model number, because I have found multiple different models with chassis codes that begin with B.
Here is a good example of a question that cannot be answered without data from other Hilgenites. I presume that if we obtain enough chassis codes from amps that are capable of being dated from speaker, pot, or other date codes a pattern will emerge. If it were purely random, why would Hilgen bother to stamp alphanumerical codes on their amps at all?
On the plus side, today I had the good fortune to purchase on eBay an amplifier that is obviously a Hilgen but that does not have the "Hilgen" badge in the upper left corner of the grille cloth or the Hilgen name on the control panel. However, it has to be a Hilgen because it has grill cloth and other cosmetic appointments that are unique to Hilgen and -- perhaps most importantly -- bears the brass "coat of arms" that Hilgen (and Hilgen alone) continued to use on some of its production bass amps. It has no Sano markings whatsoever. My present hypothesis is that Mr. Gentul built the amp before he had formally severed his legal ties to Sano, and thus could not put his name (or any other overt brand name) on it. Perhaps it was built in Jack's father's basement. Perhaps Jack wired it himself. All speculations, but plausible ones.
In any event, I believe I've found significant evidence of how Hilgen Manufacturing Company evolved out of Sano.