A WW1 era British officers field service haversack.
A private purchase item, made by
Detachable sling. - one of the snaps has broken but still works well enough - see pics
The bag can attach to the Sam Browne belt.
It's in good condition for its age, the canvas is generally clean, does not smell and is free of tears / holes.
The leather is good too with expected scuffing on the edges - see pictures.
A good example in nice condition.