Small sticker below each (they come fitted as stock pickups in a guitar) which just says "1".
I should also add here that they come as stock pickups in a guitar (strat copy) I plan to purchase second hand from some one. Manufacturer's description says they are "SD Vintage pickups" (neck and middle). Reviews I have read on the internet suggest that the vintage actually stands for SSL-1.
Will try and post pics soon.