The invention refers to a shuttering mechanism for controlling translation of a longitudinally moveable component (7) within an elongate housing (2), the shuttering mechanism comprising at least one set (2.3) of castellations on the housing (2) and at least one resilient arm (7.2) associated with the longitudinally moveable component (7), the resilient arm (7.2) having a dog (7.3) resiliently biased towards the castellations so as to engage between or behind the castellations and block the translation, wherein a respective shutter arm (10.2) is arranged alongside the set (2.3) of castellations, the shutter arm (10.2) having a number of consecutive ramped protrusions spaced from each other, wherein the castellations and the ramped protrusions have the same pitch, wherein a profiled surface is formed by the castellations and the ramped protrusions, wherein the shutter arm (10.2) is moveable in longitudinal direction with respect to the set (2.3) of castellations, wherein the shutter arm (10.2) has at least one locking position with its ramped protrusions essentially in phase with the castellations thus allowing the dog (7.3) of the resilient arm (7.2) to catch between or behind the castellations and wherein the shutter arm (10.2) has at least one unlocking position with its ramped protrusions out of phase with the castellations in such a manner that the ramped protrusions prevent the dog (7.3) from engaging with the castellations or disengage them thus allowing translation of the longitudinally moveable component (7).
