The 350 is the same height as a 520, it is 2" narrower, and 4" shorter,, may hold 5-10 gallons less water,, the stoker is the same as the 520, just set to be limited to 15# of coal per hour instead of 25# per hour..
So maybe the boiler would use $20-$50 dollars less steel, take the same time to build, have the same number of parts, stoker, and labor,, and to what effect?? with tooling up costs, it would be the same cost as a 520.. and no more efficient..
The very slight reducttion of heated surface area in the boiler is not going to make this smaller boiler any measureable amount more efficient.. If the boiler was say 25% smaller, maybe the steel savings would be worth a few hundred dollars off the purchase price..But reducing the heated surface area does not make for more efficiency, nor does a smaller stoker, not with the EFM design. Now with an AA boiler, using a 260 where a AA130 would work is probably going to cost you some unburnt coal.. but not with the underfeed pot design in an EFM.
But I'd not hold my breath waiting for EFM to start another product line up, when they are selling all they can produce, and have a bituminous experiment ongoing..