Starter Issue 74

First just bought this 74 midget .. 2 days.. I can only get it to pop-start.. I took the starter off and had it tested they said it was good, seemed good to me while being tested also (lucas) starter I think.. The previous owner had installed a push button bypass but it only clicked. so the mechanic at the shop told me to install a new one.. I did.. still only clicks from the solenoid does not engage the starter at all. IF i turn the little square thing on the starter itself then try to hit the push button the starter will grab for a second then nothing,,, I;ve never owned a MG and I have limited knowledge,, can do brakes starters and alternators on newer vehicles so.. Just looking for what I need to do to get it running... Also on a unrelated note. there is a little silver nob by the radio that when pulled out you can hear something inside the engine running ??? What is the purpose of this push/pull knob..:runaround: