Sorry slightly wrong with my answer. :-[
The 325 card *is* used as a combining amp, as you said, and is described on page 5 here http://www.classicapi.com/catalog/images/reference_docs/325-docs.pdf.
But generally an API *ACA* is as I said, 4 amps (2520's and AP2623-4 transformers) mounted on one card.
Just looked through all my schematics, and I don't have one for the ACA!!