The parts lists for '58 models dont show one but if yours is the same as a '59 model it is part #105 under the cover #102
To test it first check that it is closed
If it is place it in a pan of water a little cooler than its specified opening temperature (143F) then heat it slowly watching carefully.
It should just start to open at 143F and open fully if you continue to heat,
It helps to trap a thread by the opening flap and suspend it by the thread so that it will drop off the thread as soon as it starts to open
