I was speaking to Wayne about this just the other day. He feels that very late 1958 is when the single plug cylinder started to make an appearance. They were used on the Sportsmans, and on some of the regular bikes. I don't think they were used on th scooters.
As a side note, Wayne has a NOS single plug cylinder that is steel! This is the only one he has ever seen and doesn't have any idea when or why it was made.
Vince, what are the details on the bike you are looking at? Is it a Sportsman?