What is this hose beneath the airbox and why don't some 246's have it?

What is this hose beneath the airbox and why don't some 246's have it?

I tried looking on a parts diagram but couldn't quite figure it out

Picture is from Google for reference only

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Upon further inspection, is it the fuel return line?

If so, why do I see some cars without it?

 

It looks like the fuel return line that goes from the right side carburetor back to the fuel tank.

 

Looking at the spare parts catalog, it looks like they changed the way it exits the right side carburetor. They used a fuel connector that was similar to the center carburetor to exit to the right, instead of using a bolt that the hose attached to and exited to the rear side.

 

Ok, so this could be why it's not visible in some

I vaguely remember seeing it plugged in one Dino before, why could this be?