Gentlemen.....no mystery here ! these straps were indeed manufactured in 1916, and then dispatched to an Army Corps clothing department by the manufacturer (which is also marked upon the straps), in this case "BA III" (not II), this is 3rd Army Corps Brandenburg, Berlin (BA abbreviated from Bekleidungsamt, literally "clothing department") These straps upon arrival were marked as such. They then stayed in store as there are no more Imperial marks upon them, i.e. Imperial "Kammer" marks, "Kammer" meaning the Regiments Companies clothing closets (where they would of been stamped up by the Regiment). They could well of been attached to a backpack (since lost to time) or were just held as spares/replacements, they are easily removed and come in sizes, hence the "60" mark upon yours, so they were probably intended to "marry" up to an issued backpack which was then custom fitted to the soldier receiving it. As to the marks...."15.E./J.R.23 and J.R.4" these are not Imperial regimentals, but REICHSWEHR markings, how do we know this ?....because, the "/" mark in the middle of jr23's mark was never used before or during WW1 by the Imperial German Army, as was the font used in these markings. After the armistice, huge stocks of Imperial era equipment was seized by the allies and sold off to various armies around the world, Finland, Afgahnistan, Turkey to name a few, however some stocks were left over for the newly formed Reichswehr (restricted to 100,000 men) and issued to newly formed regiments, which bore no traditions to the old army, so J.R.23 no longer held the grand title "INFANTERIE RGT 23 " Von Winterfeldt "..... these markings could not be attributted to any SS units.
Hope that clears that up.....Prost ! Steve.