Abstract: A retainer for constraining slidable movement of a sleeve along a shaft between open and closed positions. An end cap is fixed on the sleeve's rearward end, forming a recess between the sleeve's rearward end and the end cap. The retainer is slidable in and along the sleeve. A pair of flexible, transversely opposed ribs are provided at the retainer's rearward end. A lug projects outwardly from each rib's rearward end. A spring normally biases the ribs outwardly, biasing the lugs into the recess when the sleeve is closed to retain the sleeve in the closed position. When the sleeve is closed, application of a force to move the end cap rearwardly away from the shaft compresses the lugs inwardly, compressing the ribs against the spring and overcoming the spring's outward bias, such that the lugs circumscribe a reduced circumference permitting slidable movement of the lugs through the sleeve.