Alphanumeric like all the possible characters 0-9, a-z, A-Z, or it looks like a subset, hex or base58?
It can't be base58 if it has an "l", "I", 0 or "O" on it. And given that there are letters other than a-f, it can't be hex either.
It looks like this
NNNNN-dNNNNNXXXXXXXXXXXXXXXXXXXXXXXXXX N=Number d=lowercase d X=numbers and letters
The - is in the number string in the email
Alphanumeric...Actually(not sure if this is going to help) It's 1-9 and a-z no uppercase letters
It is definitely not a private key then, because they don't have dashes in them.
Most pools let you specify the address you want rewards to be sent to. Its worth checking again if you have any wallets with this address lying around. It's highly unlikely that you used an address without saving the wallet or private key somewhere.
It's also worth checking if you have any strings that start with a 5, those might be the private keys you are looking for.