You need access to the other side of the plasterboard in order to use those back nuts. The threads look like 1/2 inch from the photo. To check that they're not 3/4 have a measure, 1/2 is very approx 7/8 inch dia. Therefore you will need some 1/2 PSP to copper fittings Having said all that, the very best thing to do is to take the shower into a proper plumbers merchants (not a DIY store) together with a photo of what the copper tails are, and get them to supply you.
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