As it turns out – the master.cf file HAD been edited, with the amavis lines completely missing.
Here’s what was gone if it helps someone else down the road …
smtp-amavis unix – – y – 2 smtp
-o smtp_data_done_timeout=1200
-o disable_dns_lookups=yes
127.0.0.1:10025 inet n – y – – smtpd
-o content_filter=
-o local_recipient_maps=
-o smtpd_helo_restrictions=
-o smtpd_client_restrictions=
-o smtpd_sender_restrictions=
-o mynetworks=127.0.0.0/8