Belay my last! Just read your post.. ahh yes I do remember this type of HIN now (VS21M79A)
(VS/21/M79/A) your boat was born Aug 1979 and was the 21st sonic built.
I think I had a few extra digits in mine, would make sense since mine is a 81 though.
Sonics manufacture ID code is ZVS not sure of you made a typo or not.