There's often a vertical line painted on the rear fuselage of the F-14, just in front of the horizontal tail plane. Usually red or black and sometimes also visual on top of the fuselage.
What is the purpose of this marking? I've tried to find out about it but my searches have come up short.
The line can be clearly visible in these two shots, for instance. Most individuals seem to have it but not all.
Photo © Lindsay Peacock - Global Aviation Resource
Photo © Babak T