Many thanks to all for the input on this question.
We certainly were not looking for an 8/6d postage stamp..

Had assumed that it would be something like a promissary note but needed more information.
Some interesting stuff in the replies and it shows that Google searching is a great thing if you know what to specify.
Had we known, or heard of, a 'Blank Bill Stamp' we would have had a result but 8/6 stamp did not cut it!
For anyone interested in these old letters there are heaps on our website (non commercial, information only) at
-/
Heaps of other stuff, stamps, booklets, thematic etc.
Thanks again and regards to all
Ron and Eunice.