This is a bit of trivia for the concours enthusiasts, something that no one will ever see.

Date Code pad printed on the fresh air valve

25/8/8 = 25 Aug 1958 --- 23/4/59 = 23 Apr 1959

In case you ever remove the screws securing the air valve plate to the shaft, those are 6BA x 1/2" round head screws, originally Phillips slot (probably not Pozidriv).

Then again, not all heaters are created equal. Here are pictures from John Russell in Halwill, Devon, UK, showing a later production heater from a 1962 MGA with the date stamp in different location. You would have to crawl under the dash and stand on your head to find this one. March 3, 1962?