The New Service never had a frame letter designation assigned to it as it was discontinued in 1941/42. It was simply the 'New Service frame'.
Pre-WW2, the OP was the '41 frame', Detective Special was the 'Police Positive Special frame'
Those letter designations were a post WW2 designation thing AFAIK.

No New Service after WW2, so no New Service frame size letter designation.
Maybe the closest thing to it post war would be their Anaconda size revolver/frame from the 90's,,that was designated 'MM'.