All I found for this was related to finance or camera shutter speeds, but I did hit paydirt chasing camera tube history. And the answer is:
The actual diagonal sensor size is approximately 2/3 of its "class" or "type", which is commonly and incorrectly listed as the sensor size in camera specs.
A good explanation is here:
Optical format - Wikipedia
"Optical format is a hypothetical measurement approximately 50% larger than the true diagonal size of a solid-state
photo sensor.
.......The optical format is approximately the diagonal length of the sensor multiplied by 3/2. The result is expressed in inches and is usually (but not always) rounded to a convenient fraction. "
Thanks for the hints. The old dog learned yet another new trick.