Here it is. It's called an anchor swivel. The P/N on the early trucks is UB3934042A.
I tried to finagle one out the other day but it didn't want to come out easy. I didn't have a lot of clearance to work with either so that was probably my problem. I'll try again to snag some in a week or so and let you know if you still need them.
Also, the lower half moon is also called an anchor swivel too - so be careful if you end up ordering a pair or else you might end up with two more lower halves.