I never had any stickers to put in log books.
As has been said, grandfather rights pertain for SEP differences (presumably SSEA too, but don't quote me on it). So any pre-JAR experience would remove the specific requirement for an instructor sign-off.
In the absence of specific guidelines, I usually put an entry of "type training for ..... and differences training for .... completed satisfactorily" with signature and number in the log book.
On occasion, I would send someone solo on grass with tailwheel, before completing the training on hard runways and wheel landings etc. To be solo, did they needed the sign-off or was it the same as instructor supervised solo for a student. Generally I signed off at that stage, but carried on with the tailwheel check-out afterwards.