it meant that most shifts were invalid due to invalid base58 characters but I did get a few valid private keys
Huh? You shouldn't have been able to do that, a random set of characters beginning with the right byte has a shockingly small chance of being a private key. It's like one in 2^32 due to the checksum.
no clue why it would be there otherwise.
Ditto the numbers "outlined" on the right hand side of the image.