The Replacement Pulley Prolongs The Lifespan Of The Simple Descender. It Is Compatible With The Upper And Lower Pulleys Of The Simple D004Aa00 Descender, And Only With The Lower Pulley Of The Simple D04 Descender. Extends the lifespan of SIMPLE descenders. Upper or lower pulley for the SIMPLE D004AA00 caving descender. Lower pulley for the SIMPLE D04 caving descender.