I've also noticed that the prices at the auction site for the Zone VI filters seem exhorbitant.

Many years ago, I bought a Wratten 90 filter, and cut a piece to fit into a glass slide mount. I've carried it around with me for probably 20 years - took it out while shooting earlier this week for perhaps the fitst time in several years.

So given the amount of use that I have for it, buying a wratten filter and devoting a spare glass slide mount makes more economic sense that paying a ridiculous price for a commercial model.