The Brotbeutelriemen was an ocher colored (early war) or later, grey canvas strap with a slide adjustment and a spring clip on each end that could hook to the "D" rings on the rear of the Brotbeutal so it could be carried like a haversack. However, unlike the WWII German Soldat, the First World War German Soldat did not have equipment suspension straps built into his tunic nor was he issued "Y" straps for his equipment waist belt. Instead, the Brotbeutelriemen would also be hooked to the "D" rings on the rear of the cartridge pouches and passed behind the neck to support the equipment belt and cartridge pouches.
and as such, their use is allowed. However, some WWII colors (such as Luftwaffe blue) are just not acceptable.