Looking at some timing covers recently I noticed a transitional style. On the left (bottom one in the second photo, back one in the third photo) is a typical 1919-25 cover, in the middle a (my guess) 1926 cover, on the right a 1926-27 cover. The middle one retains the boss for the fan arm but its never been drilled. Must have been used when the fan pulley first moved to the water outlet. On the common 1926-27 cover the boss for the fan arm has been eliminated. I also noticed the top of the oil filler spout is lower on the 1926-27 covers.
Great photos showing the differences. If you look at the 1919-1925 timing gear cover for the starter engines you will see the “factory number” T-690C in the casting. The part was redesigned and a new suffix “D” replaced the “C” for the 1926-27 cover. If you have a chance please take a look at the other two covers and see if the “factory number” is visible and if so what it says.
I believe you are correct that Ford initially stopped machining the T-690C before they fully switched over to the T-690D cover. And there would have been some time of overlap when engines were assembled using either style – with the boss not drilled and without the boss. There is also a chance that some 1926 style engines were assembled with the boss drilled but not used if they need to use up a lot of old stock to move it out. It would all depend on how well the change over timing went.
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