If buying these second hand, take working perfectly with a pinch of salt.
They may have been 20 years ago but after farting about for a day
swapping 4 units, they are all goosed.
Given the stupid torx screws where there's always at least one that's shagged so it's just a waste of time.
Tore one apart to see how it works so will post photos.
Doesn't look an easy remake but can't see how it costs 500 quid.
Specialist machinery aside they must be making over a hundred quid an hour

 

These three articles may interest you if your stripping down a metering unit...

http://www.motorsport-developments.c...AF268_tech.pdf
http://www.motorsport-developments.c...AF269_tech.pdf
http://www.motorsport-developments.c...AF270_tech.pdf

Everything you need to know!