Personally opinion, but I sort of got used too the appearance of the D600s but to me, the D6100 and D6300 locos always looked as if they were intended to have noses as well but just left them off. The flat area below the windscreens just looked un-finished. The D800s avoided this with a very simple slope. It was, I think, the case that the NBL cab was pre-fabricated aluminium alloy and the exactly same for all their diesels, with or without noses.
Another jarring, i.m.o., feature of the D6300s (but, oddly, not the D6100s) was the spoked wheels. Looked like NBL had not been following the kit assembly instructions and used the wheels from a wagon kit by mistake - as was well as forgetting to glue on the nose

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Difficult to say whether the D6100s (in original form) looked good in green as, by the time I encountered them, the colour was usually hidden by fuel and lube.oil and coolant leaks - from the roof (exhaust and rad. overflow) and seeping through the louvres. I particularly remember D6104 which also had soot and scorch/burnt off paint above some of the vent louvres and was overall as sooty black as a Black Five.
But I did like the NBL 2s, although it might not seem so from the foregoing. In their dotage they were pretty much restricted to Cowlairs bankers and one seemed to be a bit banana shaped when viewed from the front (???D6152???) possibly as a result - probably not designed for compressive forces???