How do you change the transmission fluid in a 1953 Mercury Monterey 329 flathead engine?

Most Mercury Montereys must be drained (transmission) by removing the big inverted nut that holds the filler tube in place. This is the way mine works. I have heard that some had a drain plug, but have never seen one. I have also seen suggestions that the best way was to remove the pan, but these were only guesses in the blind because you can't remove the pan without removing the dipstick tube in any case.
PS: This all assumes that you have an automatic trans. Also, unless you or someone else has swapped engines, the 53 Merc flathead eng was 255 The ford flatheads of that era had a 239 in. eng. so maybe you got the numbers inverted???????
PPS...I have a chilton manual for this period, that goes up to 1953, so email me if I can look up anything else for you, and I'll try to help