OK I will give it a stab towards the second half of 1915 flash guards began to appear. Yours has one. Beginning 1916 The Germans issued an order to stop using unit marks. They realized the British and French were using it for good intel from captured weapons. But a lot of German units continued the practice anyway. But after 1917 fewer weapons and gear were found with unit marks. I believe as 1917/1918 became more dire a lot of procedures were bypassed. Such as inspection stamps and dates. Also in 1917 the Germans issued an order to remove saw back teeth off bayonets which was largely ignored. Due to the problem of any German soldier caught with a saw back bayonet were shot. So I think your 84/98 was probably made in 1916/1917 time frame. Some manufactures made only a limited amount of 84/98's with saw back teeth. Which would also help date it. But I don't have that info.
1) Flashguards were indeed introduced in mid-1915 for these short bayonets, for other bayonets the order was issued in the autumn that year (we are re-arranging things at home as otherwise I could give you actual dates of the orders about this!)
2) The German regulations are specific: officially NO unit marks were used once hostilities commenced, as their official purpose was to identify which wapon belonged to which soldier in those units that were 'in service', as it were. However, it is clear that many were marked after August 1914 which is why as Orcutt indicates, a follow-up order (not an original order) was issued in 1916 reminding unit armourers of this. As far as I can recall it had nothing to do with any potential intel value: a captured German's unit's identity could be ascertained by his shoulder straps and/or his pay book. They continue to appear into 1918, but seem to be more usual by then on revolvers and pistols than bayonets.
3)
Unitmarking might have slowed down by 1917, but not inspection and official state / year marking which continues into 1918. So, what have we here? Well, a hint as to why this one and so many others of its type, plus a fair few 98/05's lack state and date marks, is simply the chaotic state the German armaments industry was in during late 1914-early 1915. Over a million bayonets were urgently needed (hence the 'Ersatz' bayonets), and first in line for production and distribution were short bayonets like this for use by cavalry units, which were ordered to be supplied with bayonets in - IIRC - December 1914. I believe that most of these S.84/98 and S/98/05 that lack state and year marks belong to that period. Can't prove it though!
4) The orders to stop making sawbacks and the removal of sawbacks from bayonets with them
if in front-line service do indeed date to mid-late 1917. A
Festungs unit would not necessarily have to comply with this order. I cannot agree with Orcutt that the order to remove saw back teeth off bayonets with them was largely ignored: it is true that proportionaly, more sawbacks survive than were ever issued, but that is because they were collectable.
5) There is no
proof that a German soldier was shot if found carrying a sawback. The most famous rumour regarding this states that an entire platoon of the
Füsilier-Regiment Nr. 73 equipped with sawback bayonets was shot after capture by the French, but no such incident is recorded in the regimental history (von Szczepanski 1923), nor in the memoirs of Ernst Jünger, the regiment’s most famous member. (For further information on sawbacks see my article: ‘Un engin de torture, une baïonnette à crochets;une arme blanche déshonorée’: an historical-archaeological evaluation of the Sawback bayonets of the Deutsches Heer',
Journal of Conflict Archaeology, DOI: 10.1080/15740773.2019.1730074).
Hope all the above helps!
Julian