9023236 is a good one.
Unfortunately, this is the 4th one I've tried
I can't remember the exact serial numbers of the previous 3, but my tale of woe is as follows:
First one (first UK batch, 9014xxx), had slight de-centering and trouble focussing at 50mm. Was great stopped down though.
Second one (latest UK batch, 90233xx), was total rubbish in the corners unless stopped down to F16! Focussing was good though.
Third one (latest UK batch, 90244xx), seemed same as second one, although I only tested it briefly in the shop.
Fourth one (latest UK batch, 9023236) is a good one. Better in the corners at F2.8 than the second one was at F5.6
Fortunately, the shop where I bought it (the excellent and highly recommended SRS Microsystems), displayed great patience while I worked through all the problems
Cheers,
Reuben0