May not be talking about bayonet frogs directly, but I can tell you mixed steel and aluminum hardware occurs from 1940 until 1943 in most things WW2 German. I have tornisters, canteens, straps, and various rucksacks with aluminum rivets on one strap and steel on the other, examples of aluminum washers and steel rivet heads and vice versa, and even examples of steel buckles and aluminum rollers and aluminum buckles with steel rollers. I even have equipment where there will be mixes of all steel and all aluminum hardware subassemblies on the same item. These tiny pieces of hardware did not come down a conveyor belt in a nice line, the were likely dumped into massive bins in sewing shops and utilized by just scooping out what a worker needed.
Another note is "latewar" riveting on equipment occurs much earlier than people think, riveting begins in 1942, Germany's first full year in Russia for context.