I think Russ is on the money.
Theres little to no info on the net about them but I did find this.
https://en.wikipedia.org/wiki/Borg-W...5_transmission
Looking through my workshop manuals they're giving detailed instructions on how to rebuild them but at no point do they say what the box actually is
However the MK1/2 manual is saying they're the same between the two marks.
And heres a pic from the factory Mk1 manual so, hopefully, you know what you're looking at and for.
Hope that helps.
