Results from the overnight vinegar soak are in....
This grate seemed to be aged for a few good years and was layered with what appeared to be years of cheap hamburger.
But, now we are almost ready to use.
I used appx. 3 cups of vinegar and 1 box of baking soda. I soaked the grate for about 15 hours and then quick scrubbed with a grill brush then rinsed with garden hose.
I will clean again with a Scotch Brite pad before installing into the UDS.