Bing's place in tape recording history is well known. He was the prime mover in having radio shows pre - recorded and all his shows from the start of the Philco series on 16 October 1946 were recorded, but at the outset this was to acetate disc. Tape recording of the shows started on 1 October 1947. This was still way ahead of the commercial recording studios however. 

I have seen numbers of references to the adoption of tape by the major record companies and various authorities mention delivery of Ampex tape recorders to the big studios during the course of mid 1949. 

However I do not recall seeing any detail as to the precise switch-over date for actual recordings by Decca (or indeed anyone else). 

I see that the notes to the recent three CD set of Al Jolson by Sepia, written by Christopher Popa, state that the recordings were made on 16 inch acetates up to 16 February 1949, Master No L 4902, whilst the recordings from L 5017 on 17 May 1949 were to tape. This master number series is the same as that used for Bing's recordings in Decca's LA studio, so it gives us a good lead.

It would seem to me to be highly unlikely that Decca would be inconsistent between artists in the same studio so we can conclude that the last of Bing's acetate recordings could be anything between L 4857 (Why Can't You Behave?) on 4 Jan, to L 5009 (You're All I Want For Chistmas) on 11 May, whilst the first to tape in LA at least could be anything from L 4921 (Bali Ha'i)  on 10 March (unlikely so early in view of the mid 1949 delivery dates referred to) up to L 5048 (Someplace On Anywhere Road) on 2 June. 

During the period there were some recordings made in New York (W 74948 Christmas Carols to W75003 'Way Back Home) and these could of course be different as different studios could have been receiving their new equipment at different times.

Is anyone aware of anything more precise?