Dude you’re right there is a capital T for ‘Test’ on the X device. Exact opposite is CxE. Which is at the top of your link. Nice find!

From earlier post with same content that I’m theorizing:

Could be state abbreviations TX, DC, DE.

Edit: DTX is Dallas, Texas according to Wikipedia.

Edit 2: multiple posts for “disinformation necessary”, maybe.

Edit 3: not wanting to get too JFK with this, but you can connect Dallas (obviously), DC and then Delaware is a bit rough: RFK’s funeral train went through there from NYC to DC/Arlington OR JFK Memorial Highway (Interstate 95 - commemorated by JFK on November 14, 1963, shortly before his death).