The Nylon head with the neoprene O ring ans steel "scraper ring" dates from the early 70's (bought mine new in 70 as a current model and is as above). BSA also fitted an Alloy single O ring head as well as a steel "Export" head ,again with a single O ring (at the front of the head), the Export version added 1 oz to the piston weight.
As you say the steel "scraper ring" really only buggered the cylinder up and is probably not worth re fitting. John Knibbs should have this version listed as a spare, I remember Hiller offering all 3 types as spares (that was some years ago though).