Thanks to your nudging, I am indeed learning what I had hoped to about Edward Mills' postmark, and a bit more context on the general store connection. I am still deep in following up on everything, but I thought a thank you and update were in order.
In particular, the bibliography on the New York State page on Jim Forte's website at
https://www.postalhistory.com/State...rk/index.htm led to some interesting discoveries, including the directly relevant "Star Cancels of New York State" by David Proulx published by the Empire State Postal History Society in 1990, which includes references to Edward Mills' postmark. (Overall, David catalogs 70 different versions of star cancels known to be used in New York State!) I also found another of David Proulx's works "County and Postmaster Cancels of New York State" published by Empire State Postal History Society in 1990 that includes references to Edward Mills and shows a postmark from another town that is the same formatting as Edward's. I know it may be a tall order, but I am going to attempt to find the manufacturer of the stamp used based on advertisements from the period. Proulx's works are excellent but only a few hundred copies of each were printed; Luckily, copies of these two were nearby at Cornell University. I am going to try to find if he has other relevant works that might not be widely distributed.
"Star killers" also get significant treatment in Richard Graham's "United States Postal History Sampler".
I am still in the middle of following up on all the leads, but this has already been quite rewarding and has opened my eyes to a whole universe I didn't know very much about before.
A few follow-up questions if folks don't mind: the first is really basic: what postal "class" would Edward's postoffice be, or how might I go about finding that out? I see references to fourth class postoffices, etc., but I am unclear on the exact meaning. It looks like it might be related to the income of the postoffice? In which case, would the biennial "Official Registrar" volumes contain this information? (I haven't looked at these volumes yet, but will.)
The other question is probably more far-fetched: the fact that Edward's name is on his postmark seems somewhat distinctive, at least based on my limited experience, this seems to appear in the minority of cases. This suggests to me the possibility that the device used might be collectible, and may have possibly survived the decades. I'm guessing that people might even specialize in collecting such stamps? If we assume that it may exist somewhere today, either in an archive or private collection, how might I go about finding its current owner?
(Perhaps relevant, and itself an interesting footnote, to me at least: Proulx's works are dedicated to Royden ("Dick") Lounsbery, whose widow funded the printing of Proulx's works. Dick was also a collector, perhaps inspired by his grandfather, Fred Lounsbery, who was Brookton postmaster for several decades, one of Edward Mills' successors. Fred Lounsbery is a famous name in the history of Brooktondale (as it is called today), both for the length of his service as postmaster, and the fact that rural free delivery began during his tenure. A locally famous photograph shows Fred and several other postal employees posing on the first day of RFD. It is is said of Dick Lounsbery that "one of his interests was cancels, and he had many of the unusual ones in his collection". I wonder what became of his collection?)
Sorry for the length!