I know you are using Emacs but Vim users can use xxd utility:
xxd -s <start_offset> -l <length_offest> <file>i.e.
Usage: xxd.exe [options] [infile [outfile]] or xxd.exe -r [-s [-]offset] [-c cols] [-ps] [infile [outfile]]Options: -a toggle autoskip: A single '*' replaces nul-lines. Default off. -b binary digit dump (incompatible with -ps,-i,-r). Default hex. -c cols format <cols> octets per line. Default 16 (-i: 12, -ps: 30). -E show characters in EBCDIC. Default ASCII. -g number of octets per group in normal output. Default 2. -h print this summary. -i output in C include file style. -l len stop after <len> octets. -ps output in postscript plain hexdump style. -r reverse operation: convert (or patch) hexdump into binary. -r -s off revert with <off> added to file positions found in hexdump. -s [+][-]seek start at <seek> bytes abs. (or +: rel.) infile offset. -u use upper case hex letters. -v show version: "xxd V1.10 27oct98 by Juergen Weigert (Win32)".