small pella casement crank - circa 1987

I own a house with pella casement windows. the house was constructed in 1987. i bought some replacement window cranks thru Swisco which have worked well (39-175 & 39-176 units) for several of the larger casement windows. However two of the casements are somewhat shorter than the rest and have a different crank mechanism. Rather than attaching at a point on the sash, the far end of the crank actuator arm goes into a track that runs along the bottom of the window sash. as the crank is turned the end of the actuator arm slides in the track causing the window to open or close. any idea what replacement crank mechanism i need for these smaller units?