These are only a guide, you can use what ever strapping you wish inside as long as it is not seen from the outside.
Reference material
Chest to backplate
Snap upper right corner
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Snaps posterior plate
Rivets
For Level 3, three domed head bifurcated (split) rivets approximately 5/16" (8mm) in diameter are present on the left side of the abdomen and kidney and are equally spaced along the depth of the armor and are painted white. Note that the measurements are from the center of the rivet. Rivets on both kidney and ab should line up horizontally.
Interior pic showing split (bifurcated) rivets
For Level 3, a single split rivet is present on the lower tab of the abdomen armor (cod) This can have a brass or chrome finish, but cannot be painted.
For Level 2, The thigh ammo belt is attached with one solid head rivet on each side, painted white, and located in the upper corner fastened to the lower thigh ridge. Rivets shall be cap or split style rivets. No pop rivets are allowed.
Rivets should be approximately 5/16ths (8mm) in diameter (exterior).