​Note to folks on the list - please don't give away the answer...

Will

A small piece of homework for the student -- look at 407 the magic number for the a.out file ​and see if you can explain why Ken picked that value.  Hint it is particular to the PDP-11 architecture and is not portable to other systems, although most other systems that UNIX was "ported" too continued to used the a.out format kept the magic numbers as 407/411 etc., although I know of some ports (like mine own for Magix - the Tek Magnolia system of the late 1970s) that changed the magic number to be correct for that architecture.

Clem