When I fitted the pulleys to the existing track, it turned out that the track wouldn't fit between the brackets on the architrave, due to the nylon pulley insert in the metal body protuding approx 3 mm at each end
Also the lug that engages in the bracket to suspend the track did not extend out sufficiently beyond the metal body.
I ended up cannabalising the old pulleys, used the old nylon inserts and axles, with the pulley wheels from the new inserts to enable correct fit.
Works well. The old pulley wheels had become not fit for purpose.