I saw that as well last night but was going to go look to make sure I was not just remembering wrong.
Also, the '76 was a transition year between the early '70 to '76 style round metal charcoal canister mounted to the frame, along with condensing tank behind the driver's shoulder, to the later '76-'77 plastic charcoal canister mounted to the firewall and no tank behind the driver.
I say it's a swapped-in tank too, but I have never looked at a '76 tank in depth either.
Your single hard line shaped as it is in your picture looks to me like it was a later style. Your mention of a bracket(?) on the firewall kind of follows that thinking.
Is there a metal panel in the notch in front of the rear wheel well on the driver's side in the cab? Is there a rubber grommet in the oval shaped hole just visible from the front of the rear wheel opening?
If there is a grommet, is it solid and sealed? Or is it punched with a few holes in it?
I think later '77's had that hole left out and was solid metal, but I think early ones with the new system simply got a rubber or plastic snap-in plug.
Evap came on Broncos in at least CA (but I think it was all 50 states at that point) sometime in mid-'70 model year. The aux tanks got the single small vent nipple and the main tanks got two. The fill-vents appeared some time before that I believe, but don't remember all the discussions we had years ago that helped to narrow down the range of when the fill-vent started to appear on the aux tanks.
My '71 had it originally, but the front tank in my '68 is long gone and I never saw it.
Got any pics of the area behind the driver's left shoulder?
Got any pics of that area under the vehicle as viewed from the wheel well?
Any chance you can clean up the tank a bit more and look for any slight clue to there ever being a fitting?
An early evap system would have included two fittings on the tank. A later one would have had only one, or two with one plugged/capped/closed.
Maybe some of the members here with '76's and stock tanks can post up some pics of their tank venting and their charcoal canister setups.
Paul