OK, now I feel stupid. Thank you. I'd looked up the C/M manual but the lens depicted in it was different from mine. For anyone else who has this problem, the interlock release is *not* the little tab that you can press into the lens. It's the larger, ridged flat tab, which you pull towards the camera. This feels weird because the ring on which it's mounted will kind of flex, but it's apparently how it's done.