It starts with 5 but the second character is F followed by other characters with two invalid Base 58 characters and checksum error which makes me understand that last four characters are not correct.
The one charecter that doesnt fit the Base58 chart is I.
Perhaps the format of the key is not Base58. The knowledge of wallet creation may not have been widespread in 2010, so methods for generating private keys may differ, You should try to decode your key with another format ex Base64, Base62