TCAS indication for Mode A traffic

[Specaircrew]

I notice that most up to date documents to do with ACAS II only refer to Mode A/C or Mode S transponders, there's never any reference to a 'Mode A' transponder (other than to point out that they're invisible to TCAS II). Given the information that you've provided it appears that even the most basic transponder nowadays should be referred to as Mode A/C, even if there's no altitude encoding equipment fitted, because it can respond to a Mode C interrogation by leaving the altitude field blank in the reply.