Fernando, this is an intriguing 'plug' bayonet, and interesting to read the thoughts on its origin. It seems the late Roger Evans was pretty thorough in his book, but of course it is impossible to place every possible bit of info in any single reference. Still, in my opinion, there is virtually nobody who knew the world of the bayonet like him.
Perhaps he left these little gaps unattended just so we later enthusiasts had something left to do
As I know you have looked through his book thoroughly, in addition to your inherent knowledge on these weapons, I believe we have a true anomaly here. If I might throw in a note perhaps on the more esoteric side.
You note that most (all?) of these plug bayonets with undulating blade in Great Britain were for military officers. As it seems that one of the driving forces in the remarkably long use of the plug bayonet (mostly in Spain, and likely Portugal of course) was through tradition, not necessarily use in its original intention.
In England, clearly the use of the socket bayonet had replaced use of the plug bayonet in British military long guns by the 18th century.
Therefore, it would seem that a plug bayonet might be an item of vestigial importance more as an icon of tradition, at least in English context (I believe the Spanish used them more as a traditionally styled knife for hunting) .
While it is noted by Mr. Evans that no documented evidence exists showing the British ever using a plug bayonet for hunting purposes, we might assume that it might serve a ceremonial or traditional purpose . Perhaps in the manner of the 'gorget'.
In the 18th century in England, the Freemasonry phenomenon had taken hold with the upper classes, which of course included the military officers. In Freemasonry, in their tradition, they have the case of the Tyler's sword, which in their lore is often described as undulating blade (the flaming sword as described in the Bible). In the case of knightly tales, the broadsword with undulating blade is often described, erroneously termed 'flamberge', but referred to with swords of legendary figures such as Roland.
Might this plug bayonet be 'ceremonial' ? perhaps owing to these traditional and in a sense heraldic or knightly holdings, and used in such sense.
It would seem that with the well established contact between England and Portugal in the latter 18th century would open the possibility of placing this plug bayonet in their sphere as well, even into opening years of the 19th c. as we know many English arms were sent into Portugal.
The brotherhoods and orders which included Freemasonry and others certainly transcended any geographic, political or religious borders in most cases, so seems plausible.