There are no 1977 "aluminum cents".. Most likely plated. Check the weigh again. A real aluminum cent weighs about .93 grams.. Much less than your cent
The silver planchet, if it was a US coin would have to be from 1964 (dime) or before, as the only size that will fit is penny sized or smaller. How did your 1964 dime find it's way to the 1977 penny minting area? Yes it could have been introduced by a mint worker, but if it was intentional an attempt would be made to recover it before circulation. If it was an accident, would it really survive 42 years in circulation and find it's way to you without it ever being looked at or removed from circulation? Maybe but not likely. If it was a foreign coin (not many modern silver foreign planchets) again it would have to be smaller as almost all foreign coins have different size and weights from US coins. If the silver planchet was smaller than a US cent (has to be to fit) your coin would not have full and complete penny rims, which it has. 1974 was the only year for aluminum cents, they were never issued, and it is an illegal coin to own and subject to confiscation. It's true that a number were given as gifts (congress?) almost all have been accounted for, and most have probably been destroyed. There are at least 1 or 2 known examples. There is a Wikipedia article about it.
My 2 cents: Like my avatar (1882 V Nickel), some coinage is produced and given to Congressmen/Senators for evaluation. They are supposed to find their way back to the mint, but some are retained and put into private collections (that's what happened to this coin, so I'm told). Such coinage is not produced under an Act, so it's not legal tender, so-to-speak. That's why it's illegal to own...but folks do. I'm not sure why the Fed's got so upset about the 1974 Aluminum Cents, but perhaps it's because they are fairly valuable to a collector so to discourage other honest Congresspeople from turning a side profit (I mean, how could that happen?), this case was made in to the Poster Child of Coinage and what not to do. Plenty of evaluation pieces have been slabbed over the years...so they are out there to be had.
