Greetings,
I believe that you are referring to the gas tank vent hole. It is about 1 inch or so and it is below the center of the back window. There should be a pair of small holes on either side. There is a casting that fits the hole which accepts a hose on the inside. It is aimed downward toward the top of the tank which has a corresponding hose aimed upwards. It is somewhat an odd size for todays available hose material in my experience.
Nonetheless, the outer vent hole is on the underside of the casting so as to hopefully shed water and still be an open vent to the tank. Low tech but we are talking 1948. I wasn’t sure if the 201 used this system but I do know the bigger guys (509 and such) do. Hope this helps.
I would be happy to send you pics but I am not familiar with the available tools on this site. I could email you a pic if it will help.