I can tell you that the A10 flywheel is even lighter still. To make it take the larger A12 & onward clutches involves machining a bit across the whole face of the flywheel down to the bottom of the grove that defines the diameter of the A10's smaller clutch plate.
While it's in the lathe, just take a little off the back as well & you end up with a nice little "street weight" flywheel. Well, it worked for me, & i've still got it.