An old school mechanic that I once worked with on the fire department used to mark his car with a tire crayon whenever he worked on it. He did this when he was a fleet mechanic.

He used to record tune ups, new parts etc.

He would mark the replacement parts themselves or mark near his work. For instance, I marked the date I changed the ventilation system filter in my 855R on the plastic cover of the plenum.

I have modified his system by using a magic marker as well - for instance marking the new green coolant reservoir cap that I purchased several years ago.

Anyone who sees the markings can probably understand what they are for.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks