Help Identify this British WW2 Box

Any help would be appreciated. It was sold as a British Mills M36 Grenade chest, but I am not sure. Dimensions are 5.5"x6"x18". I have attached plenty of photos of markings, and photos of holes in the box where it appears inserts may have been screwed into the box at one point.

The side photo under the metal handle has the stamps SAI TD 1943 (or maybe FSAI TD??)
Thanks for any input you may have.

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