It is a hydragas/hydrolastic suspension unit. They were used instead of a damper and spring, first used in the 60's i beleive with the Mini's etc. Continued use in the 90's in the Rover Metro and 100 series and also, obviously, the MGF. It's here if you fancy a read.
Itis obviously possible to do the conversion as i have done it myself, but it is very time consuming and although they look the same, they are not!I was thinking about this the other day and wondered what would happen when supplies dry up. I checked the suspension diagrams between F and TF and from the basic line drawings they look similar, following on from this, how easy would it be to convert to TF spec and do away with the hydragas system?