BSYS tape format - PDP-10/tenex GitHub Wiki

https://github.com/PDP-10/tenex/blob/e788ef5bcdadfc0580e5df5d9af20d6f4e736902/doc/bsys.manual#L1460 (1974) http://www.bitsavers.org/pdf/sri/arc/tenex/BSYS_TOPS20_Backup_System_Apr81.pdf