The cord-grip is a sort of fibre composition, not cardboard, and was the usual material used for cord-grips before plastic became commonplace.
AIUI the socket *must* be rated to 13A to comply with BS1363 but it's possible it was originally manufactured with the 3A MAX marking for use on extension leads rated at 3A only.
Is the heater plug cracked or was that a hair on the lens?
