In the pic I posted you can see what the rivets look like (above the door opening) because there is a seam on the front of the box as well.
I'll let you in on a secret - about 10-12 years ago I actually got my right-side fresh air box through an Ebay Australia autcion. Their cars are right-hand drive which puts their heater boxes on the left - voila, right-side fresh air vent. The ones used on the '68 S/S cars are exactly the same as the right-side ones found on mid-'60s Australian Valiants.
I had to create a separate Ebay Aus. account and it took months of checking before one popped up but I stuck to it and snagged one. It cost way more to have it shipped here to the US than the part itself. The guy who had it for sale thought I was nuts but he followed through and got it to me. I've never seen another one out in the wild, it is an über rare part. Unfortunately, the door hinge on mine is broken but the door itself is OK. Not sure If I'll ever use it since I ended up blocking the right side cowl vent opening on my car. I got one though!
A bit later I bought another one from down there but it was from a later model. That one is made of molded plastic and is not even remotely the same as the other one. The vent door/grate is cable-operated like the ones that came on B bodies. Might be a tad lighter than the older one as well. Still have that one too.
No guarantees but I will try to remember to grab the older vent box from my storage spot when I'm there and take more detailed pictures for you.
