The ones with reinforcement webs in were either Quaife shells, which haven’t been available for years, or shells made by Kenny Dreer, also not available for a long time. I sold a NOS Dreer shell only last year, there was no interest on here, so it went on eBay. I suppose there may have been others by the likes of Puma etc?
The Nourish shells I’m referring to were more popular in recent years, they were designed to take a 5 speed Triumph cluster and unit construction type inner and outer covers, and had Norton mounting points. It was a cheap way of getting a very good and robust 5 speed box in a Norton.
AFAIK the only only gearbox reinforcement upgrade Steve offered was the outrigger assembly.