9 inches long and marked with the number 1164 AB, and what may be a "MPC" logo. It has a very specific job, but I don't know what that may be. I'm thinking military use but could be way off. Any ideas?

my initial reaction when I saw the photo was that it somewhat resembles an odd "Hazet" wrench I found about two months ago in a junk bin that was designed to hold the valve spring cap while adjusting the valves on a Mercedes-Benz diesel engine.

I am stumped.

This one looks like a fun rabbit hole to dive into.

Maybe a split or spindle wrench.  The Findley company in Detroit made a similar wrench.

A lot of manufacterers have an "1164" wrench - in size 5'8".  How would a 5/8" nut or bolt look when inserted in the wrench?  Photo?

I tried searching for CMP, CMR, MPC, MRC with no solid results.  There is a vague similarity to a firearm barrel wrench but I don't think that's a good match because you have that 1164 designation.

Al

Spline wrench