Leadscrew reversing gearbox comparison: instead of face dogs on the outside of the ZAMAK bevel gears (as on later machines and shown in the gearbox at the top of the picture and the gear in the right of the foreground), this early lathe had a "half-the-circumference" dog cast onto the inside of the bevel gears - with a matching dog on the slider - so giving (because of its single location in/out setting) a "dog-clutch" that could be used to exactly pick up the thread-engagement point when screwcutting.  The disadvantage would have been a reluctance to engage quickly - and hence the change to a multi-dog arrangement that was much faster to operate.