10L is the Heavy Ten lathe w/ Large spindle bore (5C capable). Could be 2-1/4 X 8 TPI threaded, D1-4 camlock, or L-type spindle.
10R is the Heavy Ten lathe w/ Regular spindle bore (not 5C capable), that is 1-7/8 X 8 TPI spindle thread.
All parts are interchangeable, depending.... and that is open ended, depends on unit code, older or recent build. However spindle bearings between 10R & 10L and associated parts w/ those various spindles may not be interchangeable. And various parts between single & double tumbler, are not either.
If you're already replacing your reversing tumbler, have a look at PM member Zahnrad Kopf, aka American Machine and Gear Co. on the outside. He makes a really nice set of "silent" reversing gears for the Heavy 10 that run on needle roller bearings. Good stuff.