SMLE Mk.V/No.1 Mk.V = I'm with Son on this one, the No 1mkV's had a thorough, broad evaluation, ie a trial, as a potential fix for shortcomings learned during ww1, but were not accepted for general issue, and it was decided that some major retooling would be inescapable to get a really better rifle. Along comes the ...
No.1 Mk.VI = truly a small production number for trials, with some shortcomings of the no1mkV, like the fragile rear sight and 'light' barrel were addressed. Almost exactly what was needed but some refinements were still needed and along comes the ...
Enfield mfg'd No.4 Mk.I, the mass produced battle rifle in the hour of need. Heavier barrel than the no1mk3 for better accuracy, and a fully rear mounted back sight to improve the sighting. Fixed items from the no1mk6 include using a plunger on the rear sight instead of a ball bearing, and removal of the fore end checkering as being not necessary enough to justify the machining time.

Just my tuppence worth ...

I do have a worthwhile question, The no1mkV had an extra band by the nosecap. This was obviously to make something stiffer, but what exactly? To stiffen the wood work to more securely hold the barrel for better accuracy? (A common idea in target shooting)
Perhaps to strengthen the front end against the rigours of bayoneting?
If its the former, can anyone verify from experience that the no1MkV is more accrate than the no1mk3?