Article on The Internet Mini Encyclopædia
The 'reversing' brand are larger, and I think there are several shapes and sizes of the reversing brand.
Both types use the same mounting bolt pattern, so are interchangeable, but the chrome bulb holder and seal are different sizes, so you can't just say, stick non-reverse filters on bulb holders with a reverse bulb.
You also need the whole assemblies, seals and all.
A pair of assemblies with lenses is actually quite difficult to find at shows for less than 10 quid [Ed: you're better off looking on www.eBay.co.uk]
Most difficult to get is the seals between the body and the chrome. The foam lens to chrome seal is sold by one or two stands at the shows.
Join the Minilist share your joys and woes with 500+ mini owners worldwide via Minilist at Yahoogroups