Mk3 Spitfire hardtop / windscreen attachment

[1969Mk3Spitfire]

The mounting kit includes two bolts and top hat bush inserts. The hardtop has 4 captive nuts and the top of the windscreen has 4 holes, all of which will accept the top hat bushes. Two are the ones with the bayonet used to retain the soft top and the other two currently have plastic push-in covers but also will take the top hat bushes 

All 4 locations have holes in the metalwork atop of the windscreen (don’t know it’s proper name). The rear view mirror is screwed to the same piece of metalwork. I’m going to have to cut the fabric covering to allow the bolt to access, something that I don’t like doing.

Why 4 attachments options? Which pair is commonly used and why?

Many thanks.

[NonMember]

I believe the hardtop originally bolted down through the holes that the soft top doesn't use. I'm not even sure my one has the other set of nuts. Why was it like that? Well, I guess it was a case of the hard top being designed for the earlier car and using a different bolt spacing to the soft top of the TR, which was inherited on late Mk3s