I have made two MK133s I am planning to join and they both work fine.
I attached an external buzzer and LED to one of the buttons for testing. I have a box with the button and an external LED (using the 9V/50ma on SW1). The external button works BUT there is an issue-
When I connect the external button the lockout fails with the following behavior.
It allows one (and only one) other button to light the associated LED (it can be SW2, SW3 or SW4). So LD1 will light as will ONE of the others and LD5 (reset) led will stay lit. The reset button resets all as normal.
When I disconnect the external button it works fine again.