Took my first proper look around under the '67 this morning to get a number for the oil filter and crawled back out from under the car very confused. The filter sits pretty close to the RH exhaust flange, but there are two outlets from the RH exhaust manifold and they split, one each side of the oil filter. The rear facing one connects to the exhaust pipe as you would expect. But there's a front facing one which has a normal flange fitting and it's also attached to a short length of exhaust pipe which has been cut off, flattened and sealed off by welding.
Any thoughts? It's got me beat.

 

It was originally a single exhaust car that was converted (badly) to dual exhaust.

The dual exhaust car used the same RH manifold as single exhaust, but factory-installed (and correctly converted from single) dual exhaust used a special flanged block-off plate that bolted onto the manifold where your flattened exhaust pipe is. On a single exhaust car, the crossover pipe from the LH to RH exhaust manifold went under the oil pan and attached to the RH manifold. That flattened pipe is the remains of the crossover pipe.

Oldsmobile liked this arrangement better than Chevy/Buick/Pontiac style Y-pipes, though it necessitated different LH exhaust manifolds for dual and single exhaust cars.

 

Thanks for that Rocketraider. I'll take it down to the exhaust shop and see if I can get a plate made for it.

 

Actually it didn't. It's just a different pipe that goes to the drivers side exhaust instead of to the passenger side manifold.

The BBO dual exhaust cars got specific manifolds without the crossover that wouldn't fit on the SBO due to the difference in deck height.