Scammell Mechanical Horse 3 ton version
Scammell introduced the Mechanical Horse in 1933. It was originaly very basic, with no side doors. Two versions were produced, a 3Ton and a 6 Ton. Doors were later added as standard. This is one of the original 3ton tractor units without doors
Scale O gauge (1/43.5, 7mm/ft)
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more info about Scammell Mechanical Horse
https://en.wikipedia.org/wiki/Scammell_Mechanical_Horse
more 3d printed models from Recreation21 here
http://www.rue-d-etropal.com/3D-printing/3d_printed_designs.htm