Guessing some kind of military rope. It's flat, probably 3/16 x 7/16. I can't find it anywhere. Thanks-

Flat Braid or Hollow Flat Braid.........

Nova makes all sorts of roap. Hollow and flat braid look a lot like your picture. Do some google puh and look at the pictures.

That looks sorta like Amsteel hollow braid. You can buy it HERE

Awfully fancy mule tape

That's good stuff too and it's probably easier to find.

Looks like am steel to me to, but I'm no rope expert.

That is army parachute cord. It is a different type that has no core. It used to be available at army surplus stores and I have braided various tack items out of it. Horn knots,hobbles and even a 23' mecate. Great stuff!

Is this it?
https://rwrope.com/1000-lb-test-type-iv-commercial-paracord/

Looks very much like Amsteel, a hollow 12-plait, although Amsteel is rounded and this line appears flattened.

Used a bunch of Amsteel recently re-rigging my sailboat. A great substitute and improvement over wire rope for many applications. Extremely strong, very low stretch and a snap to splice.

Amsteel is not something that works well as a general purpose line. It is very expensive and has essentially no stretch or shock absorption capacity. It is a high-tech substitute for flexible wire rope.

It looks like Amsteel/Dyneema.