That was the complete set of five K4s, LNER Nos. 1993-6/8. They had been built specifically for the line in the late 1930s, notably powerful with 3 cylinders and a high-pressure 225 lb boiler, 2-6-0 and not 2-6-2 to keep all the nose-up thrust of the engine when under power on the driving wheels (I shan't say that Gresley had learned this from Swindon

), all of which aided them getting up the steep gradients, despite which with summertime (and doubtless wartime) loads they would be double-headed, likely by their older and less powerful brethren K2s. The K4s had some quite grand names of Highland Chieftains from the area they served.
The two D34s left at Fort Bill doubtless spent their time on the Mallaig extension, likely including the school trains inward from Mallaig in the morning and returning late afternoon.
The K4s would work through from Glasgow to The Fort, a considerable journey for a relatively small loco, normally taking water at Crianlarich and maybe elsewhere along the way as well. The crews didn't generally go through, but exchanged along the way to return to their home base within a shift, though they could never be quite sure where the meet would be.