into a concave position as shown in fig6 the protrusions 66 are spread outwardly to reveal relatively flat surfaces 68 which are generally parallel to the longitudinal axis of the housing 10 and are disposed in outer surrounding relation to the truncated cone portion 41 of the support head 42 as the cartridge is retracted the support head 42 is engaged by the retraction of the fingers 66 and the flexible diaphragm 14 is moved into its convex position as shown in fig7 in this position the inner faces 68 of the protrusions 66 will grip the sloped faces 44 of the truncated conical portion 41 of the support head 42 thus retaining the cartridge and extension means 18 in the retracted position in the retracted position the flexible diaphragm 14 preferably is secured in the convex position by sloped edges 64 of inner sleeve 61 although it will be evident that rearward retraction of the end 40 against the cap 14 may be employed to force the cap into the convex retaining position without the aid of additional locking means now to be described the sleeve 61 has a helical groove 60 which cooperates with a complementary thread 62 on the inner surface of the thickened body portion 57 of annular collar 12 to move the sleeve 61 within the housing in response to twisting of the collar 12 the sleeve 61 is guided along the longitudinal axis of the housing 10 by circumferentially spaced tongues 69 on the inner surface of housing 10 which are received in grooves 67 of the sleeve 61 as.