I still have my old FM 21-75. I thought it was the most useful FM I had.
FM 27-75 looks good overall but I noticed that the new 27-75 eliminated the combat tracking instruction that was in Appendix E of the old 21-75. That's too bad; although it was very basic instruction I thought it was a good introduction to the subject.
At the very least it served to make the reader track aware/track conscious, which most soldiers in my day were not.
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