Ended up dropping $205 on a new one from Video Display Corp. (
http://vdc.mybisi.com/product/21zp4) not sure if it's actually the correct tube but it has a 1 year warranty and I guess I can return it if it isn't...the product picture doesn't look anything like a 21ZP4 but I'm hoping that's just a generic stock photo. Was there ever an overlap of monochrome tube numbering?
I'm also not sure if it'll have the dag coating as it's not an "A" or "B" style. Is Aquadag something that's sold in brick-and-mortar stores or just online?