Lambda the Ultimate

I would like to track down earliest published (for citation) introduction of the terminology "abstract syntax." At some point prior to 1975 I picked up (and used) the terminology from the literature I surveyed while turning my doctoral dissertation into a monograph. But I can't find any references in my remaining old research notes.

I was mainly looking at articles by Dana Scott, Strachey, Milner, Landin, McCarthy at the time, and the then-existing literature on denotational semantics.

Additionally, also for citation, suggestions to the "best" discussion of abstract syntax, comparing it to concrete syntax.

Thanks.

Hal Levin