Very cool find! Do you own this panel?
That’s the same model of panel that NewAgeServer posted a while back in this topic: Unknown (possibly rare) Notifire FACP - #25 by NewAgeServer
It looks like the one Andrew posted was a newer version, due to the “Notifier Emhart” labeling on the modules. That unit had a manufacturing date of 1976, for reference. The unit in your pictures, on the other hand, still bears the older “Notifier Corporation” markings. This means it is post-1956, as the company had previously been known as Notifier Manufacturing Company, yet pre-1968: when the company was sold to Emhart.
In short, the panel was made somewhere between 1956-1968. Based on that, I’d say the date Jack gave is a close estimate, if not spot-on, especially due to that date seemingly appearing in the serial number.