Dick Shattock's RGS Atalanta cars were so-called because he used Atalanta chassis, or chassis parts. The bodies were just RGS not RGS Atalanta.
The Atalanta chassis looked like this:
That one has a supercharged Gough engine which Shattock wouldn't have used. I remember a Jaguar-engined sports-racer, but not this car which looks as if it may have had a Bristol engine.