So he should've been using something like this, a "snapsack", which sometimes gets a blanket tied around the outside...,
It's good for medieval up through AWI....
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BUT as you can see, that is not what this is, taken from the movie...
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What they gave him was a modified "new invented haversack" with the small straps apparently removed. It wasn't a thing until the AWI, so was out of place for the French and Indian War.
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Here's a adequate video on the pack...,
New Invented haversack being used by Fantasy Role Player...
The video haversack/pack has the weight bearing strap buckle in the wrong location....and he's wrong about back packs too.... but it does give good visual clues....
LD