What is this?

I'm looking for a violinist or other string player who knows what the notes in the last measures of this means:

It's from Shotakovich's 11 symphony in the final movement near the end. I've never seen the small stemless notes under normal notes before and haven't been able to find an explanation. I have a few guesses myself, so guesses won't help much.

If they were normal sized and stemmed I would recognize them as an alternate way to make a grace note. This is my first guess, so I'd appreciate confirmation.