By George, I think I've got it! It's newer than I thought.
Unless there was an identical older version (and the serial number indicates there probably wasn't) it's an Envelope Portfolio style number 5190 from around 1987, not from the 70s as one seller said, and with no direct connection to the Bonnie Cashin era. There's only one size listed and Coach gives measurements as 16 1/2 x 11 x 3 1/2, although there will always be variations in measurements unless a bag is perfectly rectangular and non-flexible.
It wouldn't be a Soho since Coach didn't start to use that family name until around 1993, but it's described as a lighter-weight leather. Here's Coach's catalog page from the 1987 Holiday book:
Wow Hyacinth. I am amazed. Thank you so much!