I've been watching seatbelts for early Minis on Ebay for a few years. I've only ever seen the two main styles, but there could have been other ways to mount the short end to the floor. They seem to have started off with two holes then standardised on a single hole. A lot of them have that horrible clip thing that has to hook into an eye.
I've bought a couple of sets in the last few years, but all were expensive. They are surprisingly common, two sellers in the UK seem to be slowly releasing them a pair at a time so as not to flood the market, they can sell for anything up to $400, but mine were less than that. The Britax ones usually have silver grey webbing and chrome clasps and the Kangol ones usually black with black clasps. The Kangol ones are a little cheaper. The ultimate ones are Irvin seatbelts, with a parachute logo, they were used in upmarket cars.
A possible cheaper option is to get old ones reconditioned, despite what people from NSW and Queensland think, there are places that can legally re-web them with modern webbing in multiple colours and even repair and replate the catches. You don't have to have black plastic and flourescent orange seatbelt buckles.
http://www.seatbeltsolutions.com.au/services.asp
Tim
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1951 Morris Commercial J Type Van
1955 BSA C11G
1961 Morris Mini Traveller
1969 Triumph TR6R
1977 Leyland Moke Californian