German M31 WWII Rucksacks were designed to attach to the Y-straps and they were not issued with factory made shoulder straps. (Some other models of rucks had built-in straps). This presented an obvious problem if one wasn't wearing Y-straps, so the Germans did make a variety of "field made" straps. Every division had a maintanence section that repaired equipment- which included modifying and making gear for special purposes. Over the years I've seen hundreds of WWII rucksacks- it's rare that any two had the same "alternatively engineered" straps. They were made from just about every conceivable strap or webbing in the Wehrmacht inventory.
These are copies of original straps I encountered at a friend's warehouse in Germany in 2012. They are made from cut down Y-strap blanks. His were riveted (very much period done- dry rotted and well used) but I suspected the know-it-alls would squeal so I designed ours with stitched ends instead. Neither these or the canvas straps are more or less "correct" than the other.
These will fit any German rucksack with D-hooks. Sold in pairs.