SB-16A Speedlight & F3

The operative end of the SB-16A's mechanical linkage ("feeler"), which, as the flash's lock-ring turns (half-turned, above), engages the raised tab ("ISO tab") on the film-speed adjustment ring, providing the flash module with the ISO speed set on the camera. Off-camera TTL flash connectors (AS-17 adapter, SC-14 TTL cord) have duplicate rings simply to engage this linkage.