A two piece left hand nut will have 12mm-1.0 RH threads on the inner half of main nut body. The outer half will be 1/2-20 LH threads.
A second outer nut with 1/2-20 LH threads will screw into the main body allowing you to start the engine on the ignition side when engine is mounted on the left side of a kart, usually in a dual engine or enduro kart application.
Over the yrs. I've found that my pop-off guage/pumps are only good for maybe 2 rebuilds, throw away and buy a new one. Mike, I forgot that one, because I never had to make anything to fit on that side.
The last time I attempted to purchase the G1673 and G1644 two piece left hand nuts from GEM, they indicated they did not have a future production date.
Same for the replacement outer nut G1696.
I bough every last piece of their inventory at that time.
I will have to check with Rhitt to see if they have made another run.
FYI, the only difference between the 1644 and the 1673 is the 1644 includes a washer to accommadate the deeper depth of the recess in the original MotoPlat Rotor versus the Yamaha.
When installing the main body nut, first lightly tighten by hand to make sure you are not running into the exterior left hand threads before fully tightening rotor. As a simple precaution, using a washer on either application (or two) will not hurt a thing.