Well that was a waste of time! Not surprisingly, given the number of locos involved, the RCTS tome doesn’t list boiler changes for 57xxs!
There is a photo of 5707 in Pannier Papers #3, dated October 1958. It shows 5707 at Canton shed, allegedly on its way to Swindon for withdrawal. It is in unlined black with the early BR emblem and is fitted with a top-feed fitted boiler.
Sorry I can’t be of more help
Tim T