Ms. Porter's EE Times article was brilliant, and it brought back
memories.  I thus wrote a comment on that article, and perhaps
MMD subscribers might find it of interest.

Sams Photofacts and other independent schematic services have an
interesting history.  It turns out that manufacturers in the early
days of radio were not so forthcoming with service information,
even going so far as to establish proprietary color codes for
components like resistors and capacitors.

What the Howard W. Sams Co. did was purchase popular radios from
retailers, take them back to the lab, and disassemble each one.
Circuits were traced and components measured.  Non-proprietary
substitutes for components were chosen and listed.  Several views
of the chassis were photographed and each component labeled, and
a traditional schematic was drawn.

The completed information packets were compiled into folders
containing several recently-introduced radio or TV models, and
service shops would either purchase these at the local radio parts
dealer or subscribe to the Photofact of the Month service.
Photofacts were rather costly, but nevertheless sold well; I still
have several in a file drawer somewhere, and they got me off the
hook on several occasions during my audio repair career.

Mark Kinsler - Kinsler Hi-Fi Service, 1971-1989