Hiya, in the early days of the ATS, Officers did not wear conventional rank "pips". Instead they wore these specially designed rank stars.
The first issues were Bronze as seen here, while later ones were Gilt metal. These are very hard to find as they were only worn from 1938 when the ATS was first formed until Feb 1940. The gilt versions were worn until June 1941.
This design of rank star was replaced due to the fact that ATS officers were not being saluted by the male rank and file as few soldiers recognised them as Officers due to these unusual rank badges. So conventional rank "pips" as worn by their male counterparts were introduced in 1941. I have listed the ranks here, 1938 to 40 first, followed by the post 41 rank and in brackets the equivilant male Army rank.
Company Assistant or 2nd Subaltern wore one star (2nd Lt)
Junior Commander or Subaltern wore two stars (Lt)
Company Commander or Junior Commandant wore three stars. (Captain)
On 9 May 1941, the ATS rank structure was reorganized, from July 1941 the ATS was given full military status and members were no longer volunteers. Other Ranks now held identical ranks to male Army personnel, but officers continued to have a separate rank system.