Nick –
I've looked back through Tony Griffiths' archive, and I see what you mean.
I no longer have the lathe but have identified it as matching the 1908 catalogue photo, with internal drive-shaft and leadscrew, compound slide and that elegantly arched head end of the bed. However, further down the page we find a heading, "Drummond B-Type circa 1906" – but also with the internal shafts.
'
Chris –
Whatever "M" stood for or meant, it was neither "Military" nor "Metric".
Not metric because 0.125" does not = 3mm. As few as 10 revolutions of an 8TPI screw gives a travel of 31.75mm, not 30, with correspondingly significant errors possible in any screws intended as metric but cut with the standard inch-thread change-wheels.
Metric screw-cutting from an 8TPI leadscrew on a lathe too small to cary a 127T conversion change-wheel normally demands a 63 or 32T change-wheel in the combination, even then producing approximations close only over fairly short distances. The Myford ML7 gives several metric pitches with its normal inch-pitch wheels, but the necessary compound train might not be possible on the Drummond.
Why the letter? Seems a puzzle as the 'M' was developed in the early 1920s, and not originally for the Services particularly.
Drummonds later made a version for the Admiralty, with powered cross as well as long feed, but called it the "BS". The loop over the headstock was really for rigidity but may well have found a second career as a rather hazardous hand-hold for a "tiffy" using the lathe aboard a destroyer bouncing around the Atlantic in a SSW Force 7!