As you have noted steptoe the original RX was released in 1972 as more sporting version of the Leone coupe. It was equiped with 4 wheel disc brakes and a 5 speed transmission as standard (Unlike the GSR). No 4WD or turbo in those days, just the same twin carb motor (EA63S) as fitted to the GSR. From the sales catalogues I have it looks like the RX was marketed towards the rally/race types while the GSR was more sport/luxury.
The coupe in the Leone ad on youtube appears to be a GSR not an RX. And 'ugly' is only a personal opinion!!
The RX had black body striping (which you could have on the GSR too) and a large 'RX' sticker on the bonnet and boot. There was also a small '5speeds' badge on the boot too.
Basically the RX was a 'trim' option on Leone models from the start in 1972. It was not until the 'MY' that there was 4WD and turbo as part of the package.
I note the wikipedia entry for the Leone has been updated and includes a photo of the RX. You can just make out the RX on the bonnet. The early RX (1972 to early 1977) only came in two colours a greenish blue or silver (as per the wikipedia entry) and had black windscreen wipers instead of silver like the other coupes. I believe from the 1977 facelift different colours were available. Also from 1977 the RX was completely blacked out, any items that would have been chrome on a GL coupe were done in black.