Sheldon 13" lathe Series M of 1955. This machine is a Model UM56B with the "two-lever" instantly-selectable spindle speed drive system.
The bed of the 13" lathe was appreciably deeper and more robust than that of the smaller models. The 1950s machines had improved screwcutting gearboxes with 54 rather than 48 threads, the tailstock was redesigned with a greater forward "reach" and a 3 instead of 2 Morse taper in the barrel. The old-fashioned looking stands were replaced with cleanly-designed cabinets with full-length chip trays and improved storage.

A late model 13" Sheldon on the cast iron cabinet.
By 1958s the lathes were again improved in detail. The 13/8-inch bore spindle was fitted with a much safer and more robust Cam Lock D1-4" nose instead of the threaded type, the rack-feed handle on the apron was given a distinctive, stylised appearance, a toggle lever instead of a screw helped to speed the selection of power-feeds on the apron, the screwcutting gearbox could generate 60 threads instead of 54 and the micrometer dials were increased in size and hence clarity.
Besides a series of turret, chucking, automatic and tracing lathes (etc.) Sheldon also made the quite different R-13, R-15, R-17 and VR variable speed lathes - a description of which will form the basis of a separate article..