Intake stopper?

A little background: when I got my w150, the 318 ran alright but was an absolute dog, very little power, pretty much had to floor the thing to get it to downshift on hills. It was almost like a massive bog, but felt more like it was just lacking power. Fast forward 3 months later, I got a 4bbl intake and carb on the thing, and it has gobs more power than it ever used to. The carb was new when I got the truck, and was so small that I thought it was a 1 barrel, but it turned out to be a very tiny 2 barrel. Either way, left that carb on the intake and went about my business. Last night I took the cast intake off of my other 318, it's intake was in pretty bad shape compared to the trucks old one. Took the carb off of the truck intake, and there is a silver "plug", of sorts, in one of the 2 holes in the intake, right under the carb. It's obviously much newer than the intake. No holes or anything in it, it completely blocks that port off. Probably 1 1/2 inches or so tall if im guessing. Can get picks when i get home if you guys want them. I've just never heard of this kind of thing.