The Will-Burt stokers are the evolution of the Iron Fireman stokers... Will-Burt bought Iron Fireman back in the '50's.
The reason they state 'not for Anthracite' is that anthracite coal in these underfeed stokers burns to a fine cruchy powder, and these conversion stokers typically are slid into the area in a hand feed boiler where the ashpan is normally located.
The burn pot is surrounded by a poured-in concrete-like refractory cement that seals off the lower ashpan area from the upper firebox area. When Bituminous coal is burnt, it creates clinkers.. the ash from the coal fuses into hunks and chunks that collect around the perimeter of the firepot, and daily, these clinkers must be removed. Since they are hard and can be picked up in chunks, this doesn't take much time or isn't too messy if everything is right. The person cleaning the firechamber uses a hoe, shovel, some specialized 'clinker-tongs' and maybe a small rake.
With anthracite coal burnt in the above situation, the ash will pile up on the sides, but is like powder, and is much more difficult to remove from the firechamber without disturbing the fire. You would be tempted to use a big shop-vac, but this would be pretty dangerous and also I suspect you would melt a lot of Vac-hoses in the process.
In my conversion using an Iron Fireman underfeed stoker, I had enough room to leave the ash pan in place.. I inserted the stoker through the loading door [it is 22" tall] and removed the grates from the bottom of the firepot, the anthracite ashes fall off the perimeter of the firepot and into the ashpan below... like in an EFM stoker. I don't have any refractory cement surrounding my firepot. In fact the whole stoker assembly is on wheels,, I can remove the whole unit, reinstall the grates and have a new hand-feed fire in about an hour.