I bought this counterstamp just the other day at my friends coin shop. This one is listed in Greg Brunk's book as Y-2, but the maker is unknown. He lists five other recorded examples, four on Half Cents (1811, 1833, and two on 1835's) and a fifth on an 1835 Large Cent. Mine, being on an 1826 Half Cent is a new "variety" so to speak.
This mark is probably not attributable to anyone because initials-only stamps are very difficult to pin down for obvious reasons. This, however has a few characteristics that might help in an identification. The individual star in a star-shaped depression is unusual, and is something I haven't seen very often. The punch also appears to have broken at some point, which gives it a unique look. Note the rays at 1 o'clock and 5 o'clock are mishapen and crumbled. If a similar punch is found on a different coin from a known maker, then this one is likely from the same source.
From studying the wear patterns on the coin and stamp,I would guess that it was applied in the late 1830's or sometime in the 1840's, probably no later. By the style and characteristics of the letters and symbol, it may be the mark of a blacksmith or other metal worker, though not a silversmith or jewewler. Their stamps are much smaller and more refined.
I was told that the coin was part of a collection bought from a local source (South Jersey) and not from the west coast or some far location. This matters because it might indicate that it was passed down through many generations of the same family. It could help with a possible identification. I live very close to Philadelphia, so it might be a Philadelphia piece.
Whatever the case I'll research it and who knows, I might get lucky. I'll have fun doing it in any case.