Janissaries having a spoon attached to their börk, or in their plume holder

[Druzhina]

During what period did Janissaries have a spoon attached to their börk, or in their plume holder?
An early mention of this is in Miller's The Costume of Turkey, 1802, to accompany illustrations of Ottomans after Octavien d'Alvimart

Ladle Bearer
The Janissaries themselves, also, have a wooden spoon, with which they eat their pilàv, and which they wear in their caps instead of feathers ; and they as much look upon these as a part of the military dress, as an European would a sword.

In McLean's The Military Costume of Turkey, 1813 it appears as

Ladle Bearer
In strict conformity with such ideas of military parade, the Janizaries have each of them a wooden spoon, wherewith they eat their pilau, and which they wear instead of a feather, stuck into a copper tube, which is affixed in front of their bonnets.

Miller's text was based on various sources (Baron De Tott, J. Dallaway, G.A. Olivier, M. Montague, J. Pitton de Tournefort, M. d’Ohsson, etc.)
After searching I found it in one:

Constantinople Ancient and Modern: With Excursions to the Shores and Islands of the Archipelago and to the Troad by James Dallaway, 1797:
On days of gala the janissaries wear a large felt cap, certainly of Egyptian invention, with a square piece falling down behind and covering half their back; in front is a socket of copper, originally intended to carry feathers, which they bore in honour of any signal feat in war, but lately to hold a wooden spoon for their pilàv; for a good janissary considers his spoon to be as military an accoutrement as an European would his sword or bayonet.

So if this was only a custom adopted lately, it would not have been the case in earlier centuries.

Travels of Ali Bey in Morocco, Tripoli, Cyprus, Egypt, Arabia, Syria, and Turkey. Between the years 1803 and 1807 has
The janissaries of the guard wear, as do all the Turks, a long robe, but of different colours, each according to his taste, without any other sign of distinction, than an extravagant cap of greyish white felt, the hinder part of which hangs behind and covers the back; there is a plate of metal before, which falls upon the forehead, and encloses, as if in a case, a wooden spoon of a rude shape, which each janissary is obliged always to carry with him.

Are there any earlier mentions of this practice? The plume holder would have only been called a Kaşıklik (spoon receptacle) after this became a practice.

Druzhina
Illustrations of Ottoman Janissaries

[Gunner Dunbar]

Don't know anything about janissaries, but having a spoon attached to your börk, or in your plume holder sounds pretty uncomfortable.

[AdamPHayes]

I have a nagging memory of reading that this is based on a mis-translation from the Turkish that keeps being repeated. No spoons were really involved. Cannot remember now where I read it though.

[Metternich]

I've only ever seen cited as evidence of the practice those same references you cited.  And yet, if you look at the old illustrations (thank you for including such a collection of them - many were familiar to me, but there were a few new ones for me there too), you don't see any "wooden spoons"  (albeit may of the gilt plume holders have a "spoon" shape to them).  What is interesting about the story is the connection to food - jannisaries supposedly owing their loyalty to the Sultan because he fed them.  The importance of this is emphasized by many of the jannisary officers having titles that hearken more to the kitchen than the battlefield (e.g. a colonel = çorbaci, or  ('soup cook'), and that their call for mutiny was the overturning of their kettles. 

[Druzhina]

There is a reenactor site that says the plume holder is called "Yunluk" and claims this means "spoon",  but Yunluk is from yünlü meaning woolen from yün (wool) so does not mean spoon, it is referring to a plume. Kaşık and divane are Turkish words for spoon, not yunluk.

Two of the above quotes are supposed to be eye-witness accounts, but there do not seem to be any illustrations to back this up. (There are illustrations of Janissaries with a staff.) It fits with the culinary theme for Janissaries, but it may have only been a practice in the last few decades of the Janissaries' existence, at a time when they only wore the börk (or keçe (meaning felt)) on ceremonial duties, otherwise they wore turbans (like the ladle bearer).

Druzhina
Illustrations of Costume & Soldiers

[Banderium]

During what period did Janissaries have a spoon attached to their börk, or in their plume holder?

During lunch time 

[Druzhina]

During lunch time 

No, no no - before lunch, and after lunch. 

Druzhina
Illustrations of Ottoman Costume & Soldiers