I came across these weird looking little dudes in a horrendous Trombone concertino that my orchestra is swallowing whole:
I apologize for the crummy image. I refer to the strange hybrid-looking things starting at the 4th measure of rehearsal E.
I suspect they are tremolos; specifically, slow ones. That's why there's only one beam, rather than the two or three that you would normally see.
I've generally understood that three beams = demisemiquavers, two beams = semiquavers, and one beam = quavers. Here's a visual example that I stole and cropped slightly: