Thanks BMbikerider, I had another look by Googling images of the Petri 7s lens. Though all the Petri 7 do have the M-X switch, not all Petri 7s have it, even though the manual for that model says it does. At least I know now that my Petri isn’t defective, but just another version, probably a later one.
P.S I tried posting some links of images of the lens showing the switch, but this forum wouldn’t let me.