A lot of people are (rightly!) confused about 80 characters.

I can only get there by stretching what's likely a little bit:

Original private key: 32 bytes = 64 chars hex. Version byte prefix: 1 bytes, 2 chars. Checksum (truncated sha256): 4 bytes, 8 chars.

Now we're at 74 chars.

Extra single byte 01 at the end of the raw private key to indicate that it should be used with a compressed public key when generating the address (I have seen this logic in old wallets.. somewhere .. don't forget that back in the day, uncompressed public keys were quite common) - that could add 2 more, for 76.

4 still missing - this is a stretch but, did sometimes people prepend the string with "wif:..." ? That would be exactly 4 more.