At the risk of blocking the vent hole on this burning fuze I add a few broadly related comments:
Any discussion on nomenclature and LoC attracts my interest for some inexplicable reason. have a cold wet flannel ready.
The remarks about the value of dates in the margins of LoC paragraphs, or the dates of issue with Army Orders are generally accepted. The marginal dates attempt to show the dates of significant files references associated with the LoC paragraph; for instance a Naval Ordnance file number 22190, dated 10th October 1914 contributed to LoC 17122 which introduces the numbered nomenclature for grenades ( and consumes Nos 1, 2, 3 and 4). So, it would seem that Grenade No 5 was the next number to be plucked out of the hat in late 1914. But the LoC 17122 was not issued (in the form of "issued with" Army Orders) until 1st March 1915; worse the printer's imprint on the pamphlet shows it was not actually printed until April 1915. Oversimplifying it, an interpretation of what's going on here is that the nomenclature No 5 was not ready for allocation until April 1915 by which time there is a queue of grenades ready to be numbered (and just maybe the Mills is on the top of the in-tray).
A reasonable way to establish the 'earliest date' of a munition or a nomenclature is to look on the engineering drawing. Unfortunately you have to be sure that it is the right drawing without changes to the very thing that you are trying to date. In the case of drawings for the No 5 Grenade William Mills provided a 1 to 1 scale drawing and a specification to CIW which, no doubt, were reformatted into the Woolwich style and they would have added the formal nomenclature. I cannot get to the copy I have of the CIW drawing (CIW1968) but I doubt my copy will be dated as early as April 1915.
In searching drawing registers for No 5 Grenade drawings of an early date, the series 'TW' provides and interesting tease. Four drawings TW40, TW43, TW45 and TW49 each have in their title Grenade Hand No 5 Mark I. The tease is that the register does not date them but TW65 (Bomb, ML, HE, 2-inch Howitzer Mark I/L/ iron) is dated 27th April 1915 - and the TW numbers were issued sequentially.
Lastly, on the 4th May 1915, Mills wrote to The Director of Army Contract in connection with the invitation to tender for 150,000 "Grenades, Hand, No 5, Mark I (L), Mills Pattern". Since he uses this nomenclature in his letter I think it highly likely that the ITT uses it too and also likely that the ITT was sent out in April 1915.
The bottom line on these ramblings is that I think the Grenade No 5 Mark I nomenclature was available in April 1915, it is just a question of pin-pointing the day!