Grüß Gott Bergkameraden,
Where did you get this information about a so called "M42 mountain troop Y harness?" I've been collecting original 1:1 scale Gebirgsjäger militaria for over 20 years and have never heard of such item.
For the most part, Gebirgsjägers usually don't wear Y-straps because they have the large mountain rucksack to keep most of their kit in.
Mit einem kameradschaftlichen Horrido!