Ok so the display says Circa 1954...bit far out from 57? What made the owner make this assumption? Its old,but more modern than 54...
That would have been a 'stab in the dark'.

The source thread is
HERE.
Info on the rotary valve Popes has long been very hard to find, and I'm glad to note that our esteemed Forum Historian has come up with some great info on these early models; see the link in CyberJack's post above.