The problem with the markings (there are other posts on this) is that unlike muscle car era markings there is no "key." There is very little really known about the line process for most of these trucks. This is further compounded by a door tag that has only a serial number with the most basic information on it. So while interesting to be sure, and I have had a couple trucks with similar marks there is not much to be gleaned from them. As your last line states there could be a lot of reasons for the 75. In looking at the fixed link and your number it does not look like any of the ones on trucks I have owned or some that have been shared in previous posts. The ones I have seen tend to be more thin lined and look like quick, china marker scribbles (white or yellow).

KMAC