Several years ago I was lucky enough to have the opportunity to buy my ZF 5DS25-2 GT40 box new, direct from ZF. At the time they had just 'found' 3 in their warehouse and which I believe were the last of the batch ordered by Safir for their Mk5's and subsequently also used by Mallock in their RML GT40's.
I also received from ZF a considerable amount of rebuild, parts, tech data, gear/diff ratios, etc related to the various incarnations of the series 2 transaxle.
If anyone needs help drop me a line and I will try to assist.