Other important sources of magnetic tape wound cores

These are some of the google results for searching:
manufacture of tape toroidal cores A. C. Cowen


www.magneticmetals.com
https://www.magneticmetals.com/products/tape-wound-toroidal-cores


Also on google results:
The main performance advantage of ferrite is low AC core loss at high frequency, ... and nanocrystalline tape wound cut cores, which have improved AC loss ... MPP (Molypermalloy Powder) cores are distributed air gap toroidal cores made ...
(Interesting the air gap part...)
https://www.mag-inc.com/Design/Design-Guides/Inductor-Cores-Material-and-Shape-Choices


Would the above cores make great audio transformers? Have they already been making great audio transformers?


An amazing site is:


www.vintagewindinhs.com


http://vintagewindings.com/tech%20swag/Western%20Electric%20-%20Magnetic%20Alloys%20of%20Iron,%20Nickel,%20and%20Cobalt.pdf