I think it was from a van wirt. Joe made it. That unit was sent to Wyoming to be used with a Prill stoker, which burns Wyoming soft coal. It was a forerunner to making a special efm pot for use in Wyoming. It ended up working very well after some kinks were worked out.
I tested the Wyoming coal in our 520 in Emmaus and if it was sized right, it burned just fine in our pot, without modification. Problem was, there was no breaker nearby who would make the properly sized coal. I'm not sure how the sales have been out there.