Is this a GM manifold or an 'aftermarket' replacement??

As far as I know, all 235/261 factory manifolds are the same 1955-62 in the USA.

However; The ones made for the Canadian Pontiac engines may be different. If so one may have "found It's way" into the UK.

Happy trails. \:\)

John M., I.I. #3370

"There are no shortcuts to any place worth going". -Anon