Plastic window glide

This is a small plastic clip which fits on the aluminum edges of sliding Milgard windows that moves up and down in the (aluminum) channel.  It seems that they are installed to reduce the friction of sliding and or to prevent wear on the painted aluminum.  These clips are 0.44"  tall x 0.37" wide and 0.19" thick when viewed the way they are installed.  The only dimension which would be important is the thickness of the aluminum rail to which these are inserted.  I cannot measure this since I am not near the house, but based on the plastic clip, they would accommodate an edge that was between 0.05" and 0.10" (1/10 - 1/20th of an inch) or 1.5-2.5 mm.  They simply push on to the window "rail" and there are perhaps four per window if I recall correctly.  In the two pictures which show the clips installed, the window is out of the sash and rotated 90 degrees from the normal direction of operation.