Top-notch referred in 18th century English logging to the worker who stayed nearest the top of a log as it entered the sawing mill. The importance was that the bottom workers frequently went blind from sawdust particles ruining their corneas.

Answer

The situation is probably more complex than this. The answer above refers to sawing pits, where the person underneath in the pit worked in a rain of sawdust. It's possible that "top notch" refers to loggers working on trees in the forest, rather than at the mills cutting planks, where the base of each tree was notched so that short planks could be inserted for the logger to stand on. This was sometimes quite high up the tree as loggers wanted to cut the tree so as to end up with a parallel log which made for easier handling.