Sorry for the late answer, but I was in Belarus and just now arrived.
I'm not an excellent Black-winged Pratincole observer. I have been seen only 7-8x.
That's bird strange for me, because more red on the bill ( other birds were less red at the bill base). Tail was a sligthly longer, than on the pure birds. Tail was equal long as the wingend.
Just few times breeding pure Black-winged pair in Hungary, often hybridization the both pratincoles.
P.S.: Klaus! This bird has pure black underwing color!