TOW was my first thought as well when I saw the exhaust ports, but it it is shorter than a TOW and as you mentioned the fins do not match. The dia is roughly the same at about 15cm, but this is an old range, and that could also mean variations in ENTAC, SS-10, SS-11 etc. This installation was/is used for decades as a cold weather test area by the US, and the Canadians come across to use it also. My best guess from the photos would be a remnant from the 60s or 70s, and most of the references I've looked at so far do not have the views that I need for an ID.
I'm teaching a class this morning for the EOD team that found it, I've asked them to drag it back to the unit area so that I can have a closer look. I hope to have more info later. If anyone recognizes it in the meantime any info is welcome....