If Amati was involved in any way, it'd be more accurate to say, "Amati assembled." These horns do have the standard, "Best in the World," stamp on them and the G#'s engraved, "Keilwerth," so that heavily implies that there was no outside involvement. Remember that Amati wasn't shy about keeping their name off of Tonekings.
Dating could be easy if I had found one of these with an actual serial number. Without one, I'd have to decide between a few things:
* Is a Keilwerth "King" model an "entry level" horn made with an older design, with the New King and Toneking being higher models made with a newer design?
* Did the "King" models disappear around s/n 13000 or are there newer ones?
* Does the wire key guard automagically = old? I've seen a lot of Germanic horns in the past few days and I think that it can be argued that wire key guards do not = old, but anywhere from 1900ish to 1960.
I'm still playing around with Germanic instruments, but I do *think* I can make a very good argument that there were only a couple companies that produced new designs, kinda like Conn, Buescher, Martin and others in the US did with stencils. It's harder than with US horns because you've got the *possibility* that the keywork was customized.
Research will continue.