Thanks for the replies. APC it is. Didn't find any of the machines at PHL, and agents knew nothing about them: but no immigration forms to fill (apart from the customs ones).

The queue was long, agents few. But late-joiners got syphoned off to the immigration desks that had been dealing with US residents. Just a couple of questions and fingerprints: no forms. So perhaps the machines are redundant.

A bit of a zoo at transfer security. But nothing Heathrow hasn't trained me for ...