That's unfortunate, but not surprising given how obscure the Commodore mice were even in their time.
Had a single gamer (an Intellivision owner, say) at Commodore c. 1982 advocated for even one more fire button on the forthcoming C64, and used the "We'll be able to sell Commodore-branded joysticks" argument, the entire C64 gaming landscape would have immeasurably changed.
For the two button joystick the up/down/left/right/fire are as usual but the PotX is used for the second button.