A blow mould assembly for moulding plastic preforms into plastic containers may include a blow mould unit forming a cavity, within which the plastic preforms can be expanded into plastic containers. The blow mould unit may be of a design that includes at least two parts. The assembly further includes a blow mould holder for receiving the blow mould unit. The blow mould holder may include at least two mould holder elements for receiving parts of the blow mould unit. The assembly further includes a locking mechanism configured to arrest at least one part of the blow mould unit with respect to at least one mould holder element. The locking mechanism includes an engagement member that exerts a force, in a latched condition of the part of the blow mould unit with respect to a mould holder element, onto a section of the part of the blow mould unit. The force holds the part of the blow mould unit relative to the mould holder element. The locking mechanism has an actuating member configured to release a latching engagement between the mould holder element and the part of the blow mould unit. An orientation of the engagement member is latched in a first position of the actuating member and the orientation of the engagement member can be changed in a second position of the actuating member.