The Extended Gliders and their design changed in the first quarter of 2013, so about March 2013.
The first version of the gliders required screws to be used in order to assemble the plastic gliders as well as the extended gliders. This also means that that holes found underneath the chair are smaller, than that of the later chairs.
The second version of the gliders only requires the simple insertion of black plastic gliders/feet through the application of pressure. Thereafter the extended gliders simply slide on from the black of the chair. You will also see that the wholes for the later chairs manufactured after March 2013 have much larger holes underneath the chair.