Graham,
No you haven't. It's my fault. This arose from a mod of Dave's which also appeared in Miscellany about how to tidy up the gap which occurs when a Moto-Lita wheel is fitted.
I asked him where he got the u-section and he gave me the supplier's address.
I ordered 'quite a lot' of it so have been using it for some other jobs- including beading the sharp edges around the heater box etc. Loooks good
Cheers
G