11 hours ago, wheee! said:

I have one in my car too. I will put it back in and hope it aids in sound deadening.

I have it too in my CA-spec 78, but a large piece of thin metal is a pretty poor choice for sound deadening. Cap's guess about safety regulations makes more sense, though I admit I didn't give it much thought until this thread...

In europe we didn't even have a vapor tank.

I like the maple sirup gas idea.

I guess as well security layer between vapor and interieur.

Wow...  This is interesting.  We've had a BUNCH of S30's come through the shop over the last 10 years and I've never seen that sheet metal piece.  I was tempted to say "that can't be from a U.S. Z..." - and I would have been so wrong.  Just goes to show - never say never...  Oops, how many times did I just say never.

It may have been to prevent damage to the vapour tank/hoses etc from the interior of the vehicle. In case a load shifted etc.

I believe this is probably related to rear-end collision protection (and maybe also protection from electrical sparks).  It might also have to do with protection from shifting cargo in the hatch area. Or all of the above.  Safety considerations were pretty thin when the original S30 design was laid down in the mid/late 60's.  They ratcheted up significantly throughout the 70's.

Consider:  A tank sitting behind the rear wheels that's filled with fuel vapors, connected to the fuel tank by cloth-covered rubber hoses.  Did any other cars of the era use this particular placement for the vapor recovery tank?

For possible clues re manufacturer's motivation, see:  Ford Pinto.

Instead of a destination country thing, I could easily be convinced it's a model year thing for 77-78.

As previously mentioned, the 77-78 look they look pretty much the same (from the topside) as the previous years, but in reality they are almost a different car. Completely different main body shell stampings.

Mine has a 9/75 manufacture date....

Haha! Ok, so forget that about it being 77-78. It clearly started before that!