Part number 3009C is listed as being used 1919-1926. Part number 3009D is listed as being used 1926-1927. So this Cylinder Front Cover may also be a change between some 1926 and 1927 models. Do you know what the change in the front cover was?
I'm not Donnie, but the 26-27 front timing cover does not have an important "boss" cast on the front of it, near the top, that earlier models had. It was drilled and tapped for a bolt that put pressure against the fan adjustment arm that previous models used before the fan adjustment was moved to the water outlet. The parts book may list the earlier timing cover as being used up to 26, but that was either a mistake or Ford was just trying to cover all possibilities. The Ford service manual shows both; the earlier part in the general section, and the later part in the improved car section. Other than this "boss", the two parts are interchangeable, if correctness is not required.
The earlier style timing cover with the threaded boss was indeed used on early 26 style improved engines - don't know for how long, though.
Arnie, I do not have an exact date of change, but it is possible that some very, very, early 26 model improved engines came with the earlier style timing gear cover with the adjusting bolt boss and earlier fan arm. This would be very early 26. (notice I said "possible") I believe the reason for the earlier style on very early engines may just have been a "supply and demand" thing. Some branches may not have received the parts early enough and just went with what they had, till the proper parts arrived. But right now that is my personal opinion, not based on fact. Then they came out with the fan assembly mounted to the water outlet, so no boss was needed on the timing gear cover. There are also two different versions of the outlet connection fan assemblies. They are different in the way they adjust. I need to make a trip to the "Benson" to see if I can "nail down" some of the "dates of change" I am getting a pretty impressive list of running changes made to the Improved models, I have dates of change for some of them, but most changes can only be described as very early, early, mid, or late. If I live long enough I may be able to get them all in a study of the Improved Models.
I just listed that so you could include that in possible changes. The screw adjustment for fan belt length was the first style water outlet. Later they went to the offset cam with a wrench to turn the cam to adjust fan belt length.
My October 1925 built "improved car" had the older style timer cover. This photo was taken when purchased. Water pump has since been removed.
I have seen and heard of way too many early improved engines with the 1925 and before style of adjustment, to not believe that some of them came from the factory that way. A large portion of the "running changes" were made before Dec 25th 1925. There probably was considerable "overlap" at the different branches of both styles being used, due to using up what was left, or using what they had till the new style parts arrived. If you have an Aug, Sept, Oct, or maybe even as late as Nov. 1925 Improved car I would think it possible to have either the 1925 or earlier style cover and fan bracket, or the earlier "Improved" style of the water outlet fan bracket that uses the worm screw for adjustment. But with no date of change date, it is impossible to say for sure.