Don't know if this is relevany but here goes.

The markings on the guitar (& banjo) are a relic of the early instruments such as Lute & Guitar where there was a hole drilled thro' the kneck through which a wooden "T" shaped peg was put and tightened down and was the fore runner of the modern Capo. These were placed normally at the third & fifth frets but could be different at individuals specification.

My owm instruments seem to correspond to what you all have been saying but I never use them as markers only as decoration which I believe is now their only purpose.
Hope this helps