Hi Phil,
The valve spring conversion was a tuning mod back in the day offered by Eddy Dow (for one)
Gold Stars use "Alloy" top collars (weight reduction)
From memory the bottom collars are reversed to fit the smaller diameter springs
There should be several topics about this on the forum ??
or Google brings up this as a first hit
eborbikes.com/docs/Fitting_notes/Fitting notes for Gold Star valve springs conversion.pdf
The head linked to looks to have steel top collars though? maybe a half way conversion???
Or non SR components fitted
I have heard that GS springs are also used on C15/B40 but not sure about that
John