I did a bit of \"deck lid spring\" research. It appears as if the one piece spring was used until the 40hp motor was introduced. So the two piece spring would have started in 1961. Apparently, either one can be used but for my 58 the one piece is correct.
However, there are two different one piece springs. Up to 1954 there was a larger one piece used. Then from 1955 to 1960 they used a smaller version (i.e the one shown above link).
Thanks.