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It is a silver metal screen assembly about an inch thick and 2 or so inches tall, that is glued/epoxied to the base of the air cleaner. It was glued to the raised portion of the base, inside of where the regular filter is placed. If I can, I'll post a picture of the one in my 81, but no promises as I've been having trouble posting pictures.
No one that I know reproduces them. Most people remove them and throw them away as the get clogged over time and interfere with air flow. If you really want one, try watching ebay. You're most likely to come accross one air cleaner and all. The other alternative is to post in the parts wanted section. Most likely someone that removed it will be glad to give/sell it to you.
It's not reproduced, and because it was bonded to the air cleaner, it was never serviced as a separate part by GM. To replace it, you needed to order a complete air cleaner assembly; lid, base, element, etc., at a list price of $290.00 in 82! I don't have my 81 shop manual near by, but I think there are servicing instructions in the shop manual for it. Chevrolet didn't intend for it to be replaced, just serviced periodically.
81 was the only year that used the charcoal filter. You'll have to find a used air cleaner, if want to replace this filter. It shouldn't be to tough to come up with one, there were over 41,000 81s built, and everyone had this filter. If you want, PM me, as I may know someone parting out an 81.
Yes Sir; El Camino came out with them also. Mine blocked up coming back from Texas, and I ripped it out of the breather, and pitched it at a filing station. So you can find these in more places than you think. Have a great day. Gene