I have a Boler made in the same factory as yours the next year (1974). I have been working on my 3-way fridge the past few days and also removed it last winter to fool with it.
Many, if not all, of the fridges of that era, including mine, had the electrical
controls at the back of the fridge at the bottom, so the bottom exterior vent should open to allow access to the controls. If your bottom vent does not open, then you will probably need to replace that vent for many small fridges (get the fridge first to find out for sure). The electrical and propane connections were also made at the back of the fridge, and likely still are. This is safer anyway as it is good to have a vent to the exterior for the propane connection. I would suspect that you will probably end up needing a bottom vent that opens. The top vent does not need to open, probably, although you will have to get the actual fridge to know for sure. My top vent does not open and it is not an issue.
The venting arrangement you have sounds like the same as mine. It is usually described as "barely adequate" by experts and the bigger modern fridges usually vent through the roof of the trailer. However, my original fridge with the same venting arrangement as you describe will freeze water in a few hours even though I do not have a freezer compartment (I installed a new electrical thermostat yesterday to fix that problem). So, if the fridge is working well, the exterior venting is probably not much of an issue. For a marginal fridge, better venting or a fan to help the venting is usually one of the first suggestions made.
As for mounting, my fridge has a sheet metal skirt to isolate the back of the fridge from the interior of the trailer. It is attached by metal screws to the top and side of the fridge about 1/3 of the way back, with plumber's gasket putty sealing it. The back of the shroud is not actually attached to the back wall of the trailer but rather just sits there in a form-fitted way resting against the back wall above the exterior vents. It is attached to the trailer floor along the side facing the sink, from front to back. Nothing is done to the side of the fridge that faces the front of the trailer, but that small space is sealed off by the mounting of the fridge in the cabinet. The result is that the shroud pretty much seals off the back of the fridge from the interior of the trailer. This is required both for insulation and for safety in case the propane or ammonia solution (the coolant) leaks
. I suppose that it also prevents the outside cold air from getting into the trailer too easily on cold nights. The sort of sheet metal used for house furnace
ducts would work well for this.
The fridge itself is attached to the trailer by a long metal strap which attaches through the floor with a bolt, goes over the top and down the other side of the fridge and attaches to the floor on the other side with a bolt again. This holds the fridge securely onto the floor, yet can be fairly easily adjusted or removed if necessary.
As for where you can get a used fridge, I am not familiar with Regina area used trailer parts places. In northen Alberta, which might ship to you by bus, I have been to a couple of possible sources. There is Murphy's RV Parts in Josephburg (780) 992-1830, a part time trailer wrecker which I found by asking a local monster RV dealer who did not stock my parts but was sympathetic to the plight of trying to find old Boler parts. When I was there last year, they had a Trillium
being rebuilt in the yard and a building full of old parts. In Edmonton there is Cool It RV (780) 469-4737, a fridge specialist I found in the Yellow Pages where I got a "new" thermostat for my fridge yesterday. They did not have an exact replacement (that company has been out of business for decades), but they did have a similar one that fit the space and works. Also in Edmonton, there is WK's tent trailer rebuilders (780) 469-3234, another part timer where I got a stainless steel top for my Coleman stovetop last year.