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Either J. heldreichianus (I suspect) or J. littoralis.
In favour of heldreichianus are the small seeds, unequal appendages to the seeds, somewhat narrow stems, long tight inflorescences, in non-saline damp ground. It's also more widespread. I think from photos that species also makes a larger form such as these are, presumably due to having narrower parts, although ground nutrition would be a factor.
In favour of littoralis seems to be the rather triangular capsule shape, and the small capsule.
In fact the seeds are small (0.7mm + 0.3mm appendage) even by heldreichianus standards (1-1.5mm) and the appendages just take to 1mm, whereas littoralis are 1.5-2 mm.
Whilst the capsules seem much smaller than you would anticipate.
I'm inclined to assume heldreichianus in all that and that the capsules just didn't "inflate" as you'd expect. But maybe someone with good experience might consider the problematic aspects for littoralis do fit littoralis variability...
@juncuspike @igor_olshanskyi