My opinoin only...
Rocks found in certain climates develop a patina (or varnish like desert rocks) that takes many years...maybe even hundreds of years...
Here is a rock that has a dark exterior and yet the groove appears to be fresh...
If the rock were ancient, such as from the Viking period or older native American Indians, one would expect to see a uniform color to the outer exposed surfaces. The same would be mostly true if it came from a submerged environment...
Check out a stone ax head and note the uniformity of color.
Having said that...the rock is unique...and could be a crude anchor...but most likely, of recent vintage.
Its like finding a Calvary sword dating from the Civil War that is rust free yet does not fill all of the conditions of being recently found on a battlefierld.
The true test of age is to have the rock anchor evaluated by a professional collector or curator (or geologist)...
Good luck on your research...