That would be XRX 1, a Plus 4 that, as you say, competed in the 60s with a Daimler engine. Now restored and competing in historic events, but with a TR engine sadly. To get FIA papers I believe they had to revert to the original engine, despite showing evidence that it had raced in period with the Daimler engine.
If the mods will excuse a link to another board, there's a thread
here here discussing missing Plus 4s
