This line goes all the way back and points right to where the spare would be. Im taking the box off today if all goes well so I should be able to get some pics. The line is rusted badly like it was on there since 1971! LOL It had what appears to be factory clips holding it in. (I could be wrong)
While it may have been added (I doubt it) it sure looks stock to me. There's nothing else custom on this truck so I can't imagine someone adding a Blazer tank, then removing it later.
There's no auxillary tank listed on my label either.