The Olive Drab web site has a good view of the FMTV:
https://olive-drab.com/idphoto/id_photos_fmtv.php
And a useful general manual:
https://fmtvtrucks.com/wp-content/up...0-366-10-1.pdf
Plus:
https://fmtvtrucks.com/fmtv-history/
The cab and body are by all accounts "mild steel sheet metal", presumably the same as used on commercial trucks. Probably around 16 or 18 gauge (1.6 mm to 1.3 mm), thus by TM 1-1 Third Edition about 1 point of armor.
I see the standard FMTV show plow is 11 gauge steel (3.2 mm); 20' long ocean cargo containers are usually made from 8 gauge (4.4 mm) sheet steel.
The Steyr 12M18 is similar (the original FMTV is derived from it). The cab windows are normal "import car" thickness of glass.
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Michael B.