You are correct; the alloy floorplate came somewhere at or close to S/N 220000 (or, 1958).
From what came out over the years, all of the Helstock (fullstock models were made from 1958 to 1967) rifles had an alloy floorplate.
And, finally, yes, the 1600H (Helstock) is one of the "lightweight" variants, but HVA never called it that way, it's all due to Tradewinds / Dorken Bros - Ruko for Canada.
I'll post this in the sticky.