Hi Derek,
Don Young's 5"g Railmotor designs No.s 1 and 2 were described in ME 1968/9.
No. 2 has a 4" OD boiler barrel, 2 x 7/8" superheater flues and 13 x 3/8" ordinary tubes.
No.1 has a 4 1/2" OD boiler barrel, 2 x 1" superheater flues and 17 x 7/16" ordinary tubes.
The No.3 originated in the late 1970s/1980/1981 via Harry Lumb and Steve Titley as a LSWR loco using the No.1 boiler. I have the drawings for same somewhere which I cant find at the moment but am quite sure the boiler is the same re superheater flues etc as the No.1 boiler.
The No.1 boiler is a very free steaming boiler and excellent steam producer.
It would take some doing to cram in 4 extra 7/16" OD ordinary flues to replace one of the 1" OD superheater flues.
If you fitted a set of stainless radiant superheater elements to the solitary 1" OD superheater flue I think you would end up with a very good boiler. 6mm with a return bend.
What a nice present to be given!
Cheers,
Julian