The strange is that my private key inside the multibit.key file start with "S" or "o" or "u" or "d" .... start with some strange word!!!
the only reason why you can see letters outside of Base58 character set such as "o" or letters outside of hexadecimal characters is when you encrypt your wallet file. in which case you should also see characters such as "+" and "=" since it is going to be in base64 format. in this case you will need the password for it.
my suggestion is to first figure out what format it is that you are seeing here and also make sure this file was not edited manually or after import in some other wallet at some other time.