Yeah I have burrs on one of my trees for the first time too…I think I counted 5. Unfortunately that tree is succumbing to blight. The other 2 big trees flowered too. You can get burrs without pollination, but the nuts are usually shriveled up and infertile.
Ours were from Go Native trees in PA. I’ve gotten some since then from a couple other sources…just gotta get them planted.