There must be a 1 missing from the beginning of serial number of the 50 lux. Could be paint loss. It was launched in 1959, first serial number was 1640601. Mine, in M-bayonet, is from the first production year and quite close to yours 1660042. Actually from the one shot it looks like the 50 'lux is an M-mount lens. Anyway, the 50 'lux in LTM is aparently a rear lens. According to this source only 547 were made:
Leica Screw Mount Lens Price Guide
- Carl