Pentax Q mount
The smallest interchangeable-lens system anyone has built, wrapped around a compact-camera sensor. Four bodies, eight numbered lenses, and a K-mount adapter with its own leaf shutter. Two things to know: the good lenses (01, 02, 06, 08) have mechanical leaf shutters and flash-sync at any speed, while the toy lenses have a fixed aperture and no shutter at all; and the sensor grew from 1/2.3 to 1/1.7 with the Q7, so every lens is wider on a Q7 or Q-S1 than on the original Q or the Q10.
4 bodies and 8 lenses and 1 accessory tracked. Lowest used prices pulled daily from the major used-gear sites.