What connector fits on the unpopulated 20 connection pads of the Tau2 Wearsaver board PN 250-0350-00?

J3 is the connector identifier, and the part that it is designed for is the SAMTEC TFM-110-02-S-D-A. The connector and its pads are very small, but a qualified solder technician will have no problem installing it. Magnification and a 1/32 inch or smaller tip will be needed. It is best to remove the existing solder from the board pads first. Lead-free or tin-lead solder will work.