Based on the one Helmut Weitze sold (late of the Cowan collection, I'm pretty sure), the attaching devices on eagles of this type are a pair of elongated, brass tabs (similar to a paper brad). They slide into loops on the exterior of the helmet shell.
The three round wires on the helmet plate up for auction are so long that they'd protrude past the bird's edges and be visible on the helmet. As there are only two loops on the front of the cloth-covered helmet, there is no third attaching device,of course.
The only other helmet plates I know of that have long wires like this are replicas. These wires look like the ones on plates that were being made in Poland in the last 5 or so years.
When I shared that with the seller, he simply replied, "Estate find."