Listened carefully again, Joe. It is definitely 'at.'

When the first verse is sung as the chorus, he also added "'cause" as a prefix, but only sometimes!

Cyril didn't seem to consider his lyrics to be chiselled in stone, though. Rather, he wrote them, and the then polished them down. This became more obvious lately, when an emigree brought back an early verse from 'Grey Funnel Line' which has become reincorporated.