Product review (5 stars): Just what it is.  Good replacement part
Not sure if i am reviewing the actual part or the design. But I am happy with both.  The part works great and the installation was simple enough that I would recommend anyone try it themselves.  Use some common sense in clipping the part in and slipping it in the track.  If one is replacing the part because of age, it is likely that all four of the rollers need to be replaced (should probably be sold in pairs or as a group of 4).  I would recommend replacing the plastic "pin/clip" that slips in the end of the roller guides as well, since this piece also was old and brittle (it stays in, but the clip broke off during the pin removal required to insert the rollers into the slides.)