I'd be surprised if this was term used only on one squadron, but I can't swear to that - effectively why I'm interested to understand if there was a wider policy decision behind use of the term.
And in September 1940, it seems quite clear from the records that the basic 5-man Whitley crews went off on their own. The first that occasion that I've spotted occurred on the night of 23/24 September when 3 'nursery' crews attacked Boulogne, a possible invasion port.