A "Google" search for "Canadian + "gap-filler" + radar" turned up this interesting hit:

http://wolfthewriter.com/yahoo_site_admin/assets/docs/The_Tower.159154421.pdf. Click here for a local copy.

According to the article, the fire-lookout tower pictured at Fort Frances was originally the radar tower erected at the Atikokan Gap-Filler Annex for the AN/FPS-18A radar. Judging by its design, this four-legged tower would have been a combined radar tower and fire-lookout tower like the ones at Bridgewater and Topsfield, Maine, and at Marquand, Missouri. [The radar tower at Marblehead, Ohio, used this same construction, but did not double as a fire-lookout tower (as far as we know).] The pictured tower does not show a radar antenna deck on top of the cab ... but maybe it was removed when the tower was relocated, or sometime afterwards (?).

Other Canadian gap-filler radar towers were the three-legged version like those used at most U.S. gap-filler annexes; at least, the one at Tobermory was, according to the "Recent Photos." While no Canadian gap-filler annex (other than the U.S. sites in the 64th ADD) ever became operational, quite a few were reportedly completed, including the radar tower.

Interestingly, a gap-filler annex was orignally planned for Fort Frances, ON, and was to have been an annex of Baudette AFS, MN (Z-132). That one was cancelled prior to construction, according to info from pinetreeline.org. How coincidental, then, that a gap-filler radar tower would eventually end up there!