Support has just finished setting me up with an Advanced Support exchange. Hopefully I should have my replacement Band on Friday; will update with any info on the lot and build quality. I suspect that all these initial Bands are from the same lot 1443; I haven't seen anyone report a different lot number yet.
Yep, mine broke just a few days after the warranty ran out. It's not so much the clasp as the band wore out where it connects to the clasp. I didn't even wear it every day. Microsoft quality is poor and they don't stand behind their products.