As noted in my first post, .37 is what I was attempting to use when I got the AthenaWrapper error.
It was the SSPL-Manual.exe on the SD card that got me to the rainbow screen. I tried that route due to: "NOTE 2: anyone having problems with the device entering SSPL automatically, please copy SSPLManual.exe from the SSPLManual.zip attachment to the device and run it. then once the screen is in SSPL, run RUU manually."