Your assumption would be right.
A lady aeronatical engineer, during the war, called Beatrice Shilling, devised an answer to the problem of the Merlin being starved of fuel during inverted flight. She came up with a simple solution of using a metal disc with a hole in the middle that corrected the starvation. Not the complete solution but enough to let the Merlin fly inverted. It was later nicknamed 'Mrs Shilling's orifice.'
RR later solved the problem themselves. But, one should not forget that Mrs Shilling made a very important contribution to air combat with her device fitted in the Merlin. Simple but effective.