Tiny black bees on Ragweed in flower. There looked to be a pale yellow mark on the back of the neck / front of thorax, most clearly in photo 5, and, with close inspection, discernable in photos 2 & 1 (top left bee & the bee on the far RHS). They also have the same ‘lifted plate’ appearance of the thorax as the bees in sighting 4664737 (here, most visible in photos 3 & 4). Photo 1 shows the ‘inverse tear-drop-shaped’ eyes, and the relatively short antenna. Full pollen baskets are visible in all. This could be the same Sp as at sighting 4664734 & 4664737.