I've dealt with Bulgin in the past, they sell all sorts of stuff including various sized standard pattern knobs.
When I was at Raymarine their part number system had several set up from years ago, obviously by somebody with a sense of humour.
The descriptions were:
Bulgin Knob 1
Bulgin Knob 2
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club