After years of searching and trying so many devices, I finally found the actual device that Marshall used to switch power on and off on the Marshall JMP-1 and here it is; the power switch for the Marshall JMP-1.
This is NOT an equivalent component. It's the actual switch! Check it out in the image below as it's pictured next to a faulty switch which I recently pulled from a JMP-1.
A WORD OF WARNING
The power switch is secured to the inner front chassis with a small bracket. This must be removed and fitted to the new switch as it's NOT supplied with this bracket.
Obviously made by Marshall at the factory, this is bracket is effectively a bespoke part so DO NOT THROW AWAY YOUR FAULTY SWITCH until you remove the mounting bracket.
Below shows the bracket (or collar) mounted to the switch. Once the switch has been slipped into the bracket, you just need to carefully bend four posts over the top of the switch.
And it's available to purchase here: