Here's a precancel stamp from South Vernon, Massachusetts, I observed when searching
ebay:

It's offered at this
ebay listing:
http://www.ebay.com/itm/Precancel-S...t_500wt_1081A quick search of the PSS Album pages does identify South Vernon, Massachusetts as a legitimate precancel only known as Type 807, which is correctly shown. What the precancel doesn't tell you is a bit of the history behind the little known town.
As a Massachusetts resident of more than 50 years I've always been fascinated with State history and quite frankly I never had even heard of South Vernon, Massachusetts, so I did a little research.
As best as I can tell, South Vernon, Massachusetts was located in Franklin County, Massachusetts, as part of West Northfield, Massachusetts, which is just bordering the State of Vermont, of which there is a town of Vernon, Vermont.
Even with the help of the internet, I could find surprisingly little information about this town that apparently no longer exists by that name, although it apparently did at one time as this postcard will attest:

In checking a bit further on Jim Forte's Postal History website, it seems that the area was once known as West Northfield, Massachusetts (and there was a post office there from 1849-1914) and then (apparently) the post office changed to South Vernon, Massachusetts (1914-1962) and later South Vernon Rural Station (1962-1964) and the post office closed thereafter. There was even a train station that served the area, but the name of the station did not seem to coincide properly with the name of the town (and apparently the railroad had no compelling reason to do so).
I just about gave up on my internet research when I came across this 1959 newspaper article that seems to explain some of the confusion of a small area of a town that borders two states and created a "Triple Postal District":

This article suggests that the post office was technically straddling two states and for whatever reason it was considered "Massachusetts" by its postmark (and presumably its precancel) when the post office was really just on the border of Vermont.
As I found the story quite interesting, I thought I'd share my research with other precancel and postal historians on SCF.