Hmm, just a few interesting points on the Braham paperwork and service history:
29 Oct 1965 - The Service Book lists Kingston Hill Motor Works (KHMW) as owner and the car as 'Demo'.
26 Jan 1966 - Service Book - KHMW 3,232 Miles
28 Feb 1966 - Purchase agreement for Mr Signorini (Waiter) for Vauxhall Viva Brabham GT
18 Apr 1966 - Service Book KHMW 5,664 Miles
then, the tear off portions are missing but nothing written in the Service Book until:
12 Apr 1967 - Jack Brabham Motors Ltd 15,543 Miles. No stamp but there is a signature.
Take a look at the picture, could that be hastily signed by Jack Brabham?
I've included a couple of Jack Brabham signatures I found on the internet.
Pretty inconclusive really, some of the loops are the same but it is not a perfect match. If you compare the way he writes the 'J' or the 'B' between the printed 'Jack Brabham' and the signature in the service book and the sample signatures, possibly the same hand? I guess I would need an expert to take a look. It's intriguing though, perhaps it was a joke signature put there by Mr Signorini, or perhaps it was signed by Jack Brabham.