This is a very rare ampersand structure; I only know of two fonts that have it: Monotype Script and Patria (although there must be others). So I'd also like to find out what font this is!

hhp

Goudy Old Style has a similar ampersand.

No, that's a much more common form - it's just the "conventional" ampersand but with the top open. This one is "missing" the projection from the bowl coming out of the diagonal at the bottom-right.

hhp

Cheers Hrant. It's that missing projection that I found most intriguing, because structurally there is now no way to trace this form back to 'et'. If I find the (rest of the) ad I'll post the ampersand in context of the rest of the setting.