Vintage Double Hose

When did USD/AL start putting serial numbers on single hose regulators? I have my Calypso J, bought summer of 1966, it has no serial number. I have a Calypso bought the following year, no serial number. My oldest Conshelf is 1974, my newly acquired Supreme is 78 (and I have a 77 XIV as an octo paired with it) and I have another XIV that is 84 I think it is and of course they are all serialized, both stages. Odd, but my Mistral that has been mine since 1966 and dates from the 50s has a serial number of 112974. It is a Pico Blvd model, Made IN USA.

So, why did USD see fit to serialize the double hose but not the single hose regulators?

When did that start serializing single hose regulators?

This has been answered I imagine before but I am not remembering it.

Nem

My first regulator was a single hose Calypso (model 1030) that I purchased in the spring of 1964. The second stage has a serial number in the front; no serial number on the first stage. I believe that it was built when USD was located on W. Warner Ave. in Santa Ana.
Brian