From what I can gather, in 1957 Cessna came out with an extended air intake that was screwed on to the lower cowl and added about 16” and moves the air inlet point from lower down on the cowling to about 6” below the front lip of the cowling. I’m sure many of you have seen this. My 1958 came with one but it wasn’t mounted on the plane. It’s sitting in the hanger. I’ve read it gives you increased manifold pressure by as much as one inch. I would be more interested in mounting it just to keep more debris out of the intake and keep the filter cleaner.

However, when I showed it to a mechanic on my field, he was baffled. He went straight to the IPC and then proclaimed “That’s not in the parts manual. It’s not a legal part!”. Sure enough, I couldn’t find it in the manual. Anyone out there know the real story on these intake extensions? Here’s a pic with the extension installed...they are not very common but you do see them out there on occassion.

That was a Service Letter from Cessna for the dust kit for the 180/182. I took mine off years ago to allow air from the better part of the prop to hit the intake. You can find all the SLs and SBs at cessna.com
You need to find a mechanic who has heard of things like SBs and ADs that modify an aircraft's design. This guy seems to have only heard of the IPC.

Thanks 180 guy...that’s what was looking for. Oh, he’s probably heard of them but didn’t think of checking there,