I rang Coxy last night...no joy...he confirmed that I'd dealt directly with Dave Green over the purchase of the motor and box, so he has no paperwork for any of it.
Bugger!
Neither have I.
I paid all bills for the bus from my personal account; and all papers relevant to it were filed in a separate folder.
Then...I wonder...did I for some reason pay this account from our business account?
A quick flick through my diary said it would've been in the last quarter of 2011.
She Who Must Be Obeyed handles all the business accounts, so She dug out the relevant files...and there, on 21st December 2011 was an invoice and receipt for...NV4500 man. 5sp. trans.
Yeeha!
My account must've been running a bit low at that point for some reason, and we'd paid it out of another account.
But the mystery is solved.

So thanks for the input people; I think I can order that Gearvendor trans now!
And pay for it out of my personal account this time!