Transfer screws are what you need, available here and probably lots of other places too.

Rob

Links to https://www.arceurotrade.co.uk/Catalogue/Measurement/Punches-Scribes/Transfer-Screws

This is the way I do it. The chuck will be held concentric to the backplate by the register you turn on it so make this as accurate as possible. You will need to measure the diameter of the fixing screws and the diameter of the backplate you have made. This may be exactly the same as the chuck already or you may have made it slightly larger for turning off later when the chuck has been fitted so take this into account. With a caliper measure the distance from the side of the chuck to the outside of a fixing hole and add to it half the diameter of the fixing screw and then double the result. Subtract this result from the diameter of the chuck and you have the PCD of the fixing holes. You can then measure from the edge of the backplate, make a mark and from this mark scribe a circle with a sharp pointed tool the PCD of the fixing holes by pulling the lathe round by hand. You can then scribe a horizontal line through this PCD, index the lathe round one third and again one third if it is convenient on your machine. It is on a Super7 or 7R with the 60t bull wheel, or you will have to remove the backplate to a rotary table or other indexing device for marking. You have a little leeway with the size of the fixing holes when drilling because the backplate register will hold the chuck concentric to the lathe axis.

Assuming ODs are similar enough, make a paper template for the holes from the chuck as you would for a gasket, then transfer to the backplate and mark out? For belt and braces, check measurements as above.