[Jim McDougall]

These are way outside my field as well, though I have touched on them on occasion over many years. One key writer here, Louis Pierre Cavalliere wrote a wonderful paper on them it seems about 15 years ago, but I cannot find copy.
One of the reasons for slow response, in my case, is going through as much material as I can find to answer as much as possible. In the mean time I always hope more informed people can add notes, which here they have. I think the latter 19th century pretty much right on this one (very nice BTW!).

I think as Kubur has noted, the blade is one key point in recognizing older examples. This one is particularly notable having a blade with rib, which is quite unusual as it is of course heavier. I have understood that newer ones have quite thin blades, and often without the distinctive bevels on the half of the blade distally .

The scabbard also: older ones have the key decoration on the outer side only. The fretwork typically seen here on the displayed or outer side often is in motif incorporating the 'five' (hand of Fatima) in crosses, which very much aligns with one of the primary significances of the design of these Maghrebi/Berber daggers, its defense against the evil eye.
The shape of the blade represents the boar tusk, an animal known for its defensive aggression, as well as regarded to defend from the evil eye.

The shape of the pommel is of course typically termed 'peacock tail', and while often characteristic of koummya, however there are examples without it. I recall once being told the form may have derived from the Italian cinquedea, however the crescent type pommels were known in Islamic hilts as early as 16th century, and in Middle East earlier.

The ancestry of the koummya as far as I have found seems early 19th and perhaps slightly earlier but I have not ever seen 18th century examples.
Most vintage examples seem around 1850s into 1920s, but it seems that these traditional daggers are still being made for authentic wear, despite of course the masses of them commercially produced for tourists.
The newer examples have the scabbard decoration both sides if I understand correctly.

The baldric holes are also a point of reference as has been noted, and wear in these signifies years of wear. Fresh, tight and unmarred holes of course note the obvious.