As far as
what it is, I'm 100% sure that it is exactly what Ed said, i.e. a talk set (aka self-powered butt set) for use on 2- and 4-wire lines. "Talk set" is the most correct term for it, because it includes no provision for signaling (ringing). There is really nothing else that it can be, given the internal components. It is a very standard design.
As far as
who made it and why, I'm 99% sure it was made by Northern Telecom for use by Phone Company technicians, meaning the customer (I assume) would have been one of the Canadian Phone Companies of the 1960's era.
Somewhere, in some dusty pile, there may be a printed "Practice" covering this unit. Living in the US, I've never even seen the Canadian equivalent of a BSP (Bell System Practices), but I have to assume that they do exist. I don't want to send you down another road to nowhere, but if you can find out that they do exist, and that an archive of them exists somewhere, you could get lucky. [Insert more disclaimers here, such as "Not Responsible For Hair Pulled Out In Frustration"].
The fact that it turned up at a lake cottage is interesting. It could have been used as an intercom or in conjunction with an Army style field phone, but this would have been unlikely given that it has no provision for signaling (outgoing or incoming). If the location was really remote, someone may have been using it on a phone line with a separate ringer. So-called "Toll Stations" were often insanely long (100 miles or more) and hand-cranked ring generators and local battery were the norm. The four-wire capability would have been superfluous in this case, because (I think) most Toll Stations were actually single-wire (ground return) circuits. All of my conjecturing in this paragraph relates to the "second life" of your phone - it definitely started out life as a piece of portable test/maintenance equipment and only went on the see the Great North Woods after it retired from that gig.
Jim
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Speaking from a secure undisclosed location.