So I just recently discovered that there are quite a few sellers offering Tesla 18650 cells on taobao, and one listing even sells around 3000pcs/month. They are advertised as NCR18650B replacements with a lower ESR, and go for cheaper than the listings offering "genuine" NCR18650B. The Tesla cells go for around 13 yen (2 USD) whereas the Panasonic ones go for 20 yen (3 USD).
I got a few (8 cells) to play with and do charge/discharge tests. There are no markings on the cell, allegedly they come without plastic wrapping originally and the green wrapper was retrofitted on. The charge capacity measured to be ~3250mAh and discharge capacity ~3150mAh. Variance between cells was low at around +/-50mAh. The charger reports an ESR of ~10mOhm when fully charged. During discharge I glanced at the cell voltage and mAh discharged so far once in a while, and compared to the NCR18650B datasheet. It seems the discharge profile is almost identical to the NCR18650B.
There are also 21700 cells listed, I might play with those once I can get a charger that it will fit in. I do wonder how these sellers got their hands on the cells, or if they are actually pulls from Tesla battery packs.
EDIT: Added photos