It's Ugo De Rosa, not Hugo.
Ugo was probably still building at least a few frames into the 90's - I believe he personally built some of the aluminum ones. But that doesn't mean he was building all of them even in the 70's.
De Rosa didn't use serial numbers for most of the "vintage era." Certainly not in the 70's or well into the 80's, either, AFAIK. Someone else may know when SN's start showing up on De Rosas. My hunch is that your "1A" is in fact not a serial number.