You are trying to do it the old way, in your table your using the on_off but in your signal script your using state ,
Put your signal script back to how it was and use the already created table in the startup script and a signal like this
[mc.ISIG_INPUT63] = function (state)
if (state == 1) then
mc.mcCntlFeedHold(inst)
else
-- do something else or not
end
end,
DazTheGas