I don't know if they did or not Richard, but I bet you are correct. As I understand it, the majority of A10s are running a balance factor of about 55% of reciprocating mass. Don't ask me for the maths that go into that. Way beyond me. But they must have had some sort of static figure in their heads at least, surely? Otherwise, what do you do with the blanks? How much do you zap off when finish-machining the things? And what are those radial drillings to be seen in some crankshaft webs?
That said, when fitting a prototype brand-new crank to another bike, I did splash out for a dynamic balance, and I think it was worth it although i never ran it 'unbalanced' so dunno for sure. The difference between left and right hand sides, dynamically, on initial test was significant apparently (nearly an 'ounce-inch'), but was reduced to 0.11 'ounce-inch' (way below the T&L yardstick 0.5 oz-in). Now, I don't know what that really means - but having asked for a balance factor of around 60 - 63% for that particular engine (using expensive modern rods etc I should add), I got 61% and she is smoother than my A at cruising rpm. Which, given my A is a standard iron Flash setup with 7.25:1 pistons etc and is inherently quite smooth anyway, and the other motor is a CSR-spec 650 AJS (not known for 'smooth' always), is pleasing. Differences in gearing and absence of tachometers make any further comparison odious, but I got what I wanted with the balanced crank - smooth as you like at the road speeds I want to run it at. (About £145 iirc for the balancing job.)
The % factor applied - as I understand it - really only determines at what rpm you get the inevitable vibes that go with 360° parallel twins. For pootling about, it makes no odds at all. For pushing on, a higher factor - eg 70% at the high end - is supposed to make things go tingle-free at Muskrat speeds - but I don't know how true that is because for those speeds I ride something else! (I didn't have the crank on the A dynamically balanced when it had attention a couple of years ago - didn't seem worth it to me either as it was quite a smooth engine before and I wasn't changing any key components.)
I would post the data sheets for the balanced crank here - but I can't because the scanned files are too large and I don't know how to shrink them. If anyone wants the spec sheets just as an example, happy to send them attached to an e-mail.
Cheers, Bill