While GNU tar inherits this from the original tar, it doesn't seem to be something intrinsic to the PDP-11 or DEC given that the earlier Unix 'tap' archival sys

While GNU tar inherits this from the original tar, it doesn't seem to be something intrinsic to the PDP-11 or DEC given that the earlier Unix 'tap' archival system uses 'plain binary numbers ("base 256")', not octals. Here's the V7 tar.c: https://github.com/dspinellis/unix-history-repo/blob/Research-V7-Snapshot-Development/usr/src/cmd/tar/tar.c https://github.com/dspinellis/unix-history-repo/blob/Researc... and it's documentation: https://github.com/dspinellis/unix-history-repo/blob/Research-V7-Snapshot-Development/usr/man/man1/tar.1 https://github.com/dspinellis/unix-history-repo/blob/Researc... The C code clearly shows the octal (I've never used "%o" myself!): sscanf(dblock.dbuf.mode, "%o", &i); sp->st_mode = i; sscanf(dblock.dbuf.uid, "%o", &i); sp->st_uid = i; sscanf(dblock.dbuf.gid, "%o", &i); sp->st_gid = i; sscanf(dblock.dbuf.size, "%lo", &sp->st_size); sscanf(dblock.dbuf.mtime, "%lo", &sp->st_mtime); sscanf(dblock.dbuf.chksum, "%o", &chksum); Now, older versions of Unix supported DECTape, going back to the first manual from 1971 (see the "tap" command at http://www.bitsavers.org/pdf/bellLabs/unix/UNIX_ProgrammersManual_Nov71.pdf http://www.bitsavers.org/pdf/bellLabs/unix/UNIX_ProgrammersM... , page "5_06", which is 94 of 211 - the command-line interface is clearly related to tar). Here's the tp.5 format description from V6 at https://github.com/dspinellis/unix-history-repo/blob/Research-V6-Snapshot-Development/usr/doc/man/man5/tp.5 https://github.com/dspinellis/unix-history-repo/blob/Researc... DEC/mag tape formats ... Each entry has the following format: path name 32 bytes mode 2 bytes uid 1 byte gid 1 byte unused 1 byte size 3 bytes time modified 4 bytes tape address 2 bytes unused 16 bytes check sum 2 bytes These are in essentially the same order as the "struct file_header" for tar shown in the linked-to essay. But you can see at https://github.com/dspinellis/unix-history-repo/blob/Research-V6-Snapshot-Development/usr/sys/ino.h https://github.com/dspinellis/unix-history-repo/blob/Researc... that mtime is an int[2], so 4 bytes (yes, 16-bit integers; confirm with https://github.com/dspinellis/unix-history-repo/blob/Research-V6-Snapshot-Development/usr/sys/inode.h https://github.com/dspinellis/unix-history-repo/blob/Researc... showing i_mode is an integer). Which means this older "tap" DECTape support uses 'plain binary numbers ("base 256")', not octal.