Machine build dates?

A friend of mine would like to know the build year of the following Cat tractors he owns.
These are:
D6C serial number 17R1297
D4 6U8543
D2 5U11401
D2 4U6484

Would be appreciated if anybody can help out. The serial number guide I have only gives the year that production first started for each model series.

Thanks'
John

My Book shows the following
D6c 17R1297---1975
D4 6U8543----1953
D2 5U11401---1952
D2 4U6484-----1954

Cheers Al 👋

Thanks for this info Al. I have passed it on and was greatfully received

Regards,
John