RS 1624, test 52304, was tested in 1955. It stayed in Britain.
RS 1630, test 52624, also tested in 1955, was exported to the States. It's an 8" RS.
RS 2578, test 82433 - which would make it late 1972 - is the latest one I've been able to find (so far).
I've always though that the Green Lane...