M.1893, M.1907 oder M.1915 Feldflasche (Canteen)
The typical canteens used by the Germans during the war were the M.1893 aluminum canteen with cork, the M.1907 screw-top Feldflasche and the enameled steel M.1915 model Feldflasche with cork.
IR63 recommends the M.1907 model as it is available in a decent reproduction. We would allow the M.1893 but discourage it due to the collector value and rarity by late war. M.1915 models are ideal, but finding and restoring usable examples will require some effort and skill. Typically finding canteens with the enamel intact is difficult at best, some have used beeswax to seal the interior or even brewer's pitch. You would also need to find an original, rebuildable cork top as reproductions are not available.
The M1893 Feldflasche was an aluminum canteen with a cork topped with an aluminum "pull". The cover will be in light brown wool with a full leather harness.
The M1907 Feldflasche was made from aluminum with an aluminum screw top with a cork or leather washer in the top as a seal to the canteen. The cover will be in light brown wool (although Feldgrau and Steingrau wool covers are acceptable) and a full leather harness.
The M1915 Feldflasche is actually a variation of the M1893 Feldflasche, although due to the shortage of aluminum, the M1915 was first constructed of tinned steel, but the tin wore away quickly and rust in the water was a problem, so construction moved to enameled steel. The basic canteen shape remained the same as the M1893, but the cork design changed to stamped steel caps instead of the aluminum pull. These canteens would have covers made from Steingrau or Feldgrau wool, brown corduroy or even ersatz papercloth, with full or simplified leather harnesses.