I always considered the inspectors mark as a stylized broad arrow, and the IS to stand for "inspector services". The Alpha numeric underneath is would be the identifier for the technical inspector.
The arrow is, of course, found on everything from the 50s weapons, the 50s produced webbing, vehicles, and I believe aircraft parts. I am pretty sure I have seen that similar stamp on inspection tags from the 50s and 60s. Of course, by the 50s it meant the item was inspected, rather than as a property mark like the old C/l\.
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