Hi,
Fred wrote:

As a point of reference on the Knight 600 series, I was told by a
Triplett engineer (Bluffton, OH) in the early 1960's that the three
major kit companies Heath, Knight, and EICO received permission
approx. late 1953 or early 1954 (not sure of the date) to use the
circuit design from their early 3413, as Triplett was about ready to
release their model 3413A by then.

That's very useful information. Do you mind if I repost that to the
radio-collecting newsgroup, rec.antiques.radio+phono?

I expect that what the kit outfits really wanted, was the test
information, as none of them could possibly have afforded to buy a
half-dozen samples of all tubes being produced and assign an employee
to go through all of them and come up with test data.

This also explains why all the kits used the same circuit.

73, Alan