Never found a definitive answer to that. Old dealer I knew back when I was young told me the tall cap didn't come out till the 40 came along. He was a dealer ages before they came out and still had one with a front end loader on it he used in the shop and on field repairs. I have a parts manual from 47 that shows the short cap. You would need one from each year they were made to see if the change actually came during M production. I do have M literature with pics showing a tall cap.
I have the short cap on my 49's. Just like the look better. One from a 65 F100 stepside is identical.