Interesting. The train being an H6 (it appears to be one anyway), I don't think it's a relic from the 60's, although I think I may have seen old pictures of G1s and H1s with that roll sign. Vague signs like that were pretty common. St. Clair cars in the 1970s used to just have St.Clair-SUBWAY on their rollsigns. The old "via Downtown" signs were a hold over of the coil system (those old flip-up signs that are still in the stations) that would change their destination based on the position of the coils on the front of the approaching train. You'd have short turns at Eglinton, or Museum (and St. Clair West, which is still a scheduled short-turn). And as Sean said, they are a holdover from the interlining era as well. This train may be preparing to short-turn, but my guess is those signs are being put in place on Bloor trains in anticipation of the diversions through Lower Bay next month.
Personally, I'd love to see the return of "Garage" on the buses, rather than the namby-pamby, Sorry, not in service. Bleh.