The Mk. 1 "B.S. Type" was introduced for Admiralty use in 1916 and included a range of modifications to improve its ease of use on board ships as they corkscrewed around in heavy seas. It was closely based on the standard "B-Type" machine but fitted with a power-feed apron with both longitudinal and cross feeds driven from a shaft fitted below the leadscrew  and geared to its left-hand end.   
In order to make the assembly work correctly the simple full nut that connected leadscrew to apron  was replaced by a proper double clasp nut. A saddle traverse by hand-operated rack and pinion was also fitted to the B.S. and a tumble reverse for the leadscrew drive attached to the slotted bracket on the headstock front - a bracket that normally carried a stud to mount the extra changewheel required for left-hand screwcutting. When the tumble-reverse unit became available on the B.S. it also entered the accessory lists for the standard machine - however, the full B.S. specification was only available for Drummond B and M Types, and Myford M Type, destined for naval use; it was never offered on the civilian market nor, so as far as is known, never featured in any publicity literature. On the very rare Mk. 1 BS the power-feed selector arm worked across the front of the apron whilst on much more common Mk. 2 it was positioned to hang downwards and indent into slots machined into the underside face.
"Admiralty" model contined here