Those SMS standard controlers may be used alsmot unchanged on a CPC if the software allows to redefine controls.
The only change is that the button1 (button2 on CPC) is button 2 1 on SMS.
but So you may use only one button, the use of 2 buttons would need a "re-wiring" solution (a "flexible" modular adaptator may be fine) so it can't always fit unchanged on Plus range.
It seems the "Power" button on SMS controlers is used when the controler is fitted with AutoFire option (needs to be checked) or when using the Lightgun.