Yeah, looks like some sort of 16-bit checksum, with the first hex code being the lowest byte. If you read them this way, you see they're exactly 1 bit different:
\x00\xfc -> #FC00 \xff\xfb -> #FBFF
\xdc\xfc -> #FCDC \xdb\xfc -> #FCDB
I'd say try again with 113 or 133 in the filename, instead of 123 - need a bit more evidence.