Dan,
I think manifold covers for all models are a very rare item on your side of the pond, I guess many got discarded when the tractors were run on gasoline with you having such low prices per gallon compared to other parts of the world, like us here in UK all tractors had to run on Kerosene, or TVO (Tractor Vaporising Oil) a much cheaper fuel than our petrol.
Over the years I have had two batches of 12-20 / PA and CH manifold covers cast here in England, they have gone in all directions of the UK and even carried a pair across in my hand luggage on the plane for Dennis Brown, so they must be scarce. I only have one set left which I need for my single front CH.
Happy hunting,
Malcolm.
Malcolm