It sound like you have extensive research on the locomotive in question so you may already have seen this info:
it shows jerseyfield lumber co. owning 3 shay locomotives s/n 2758, 974, and 874
it shows jerseryfield lumber co. only owning 2 locos s/n 974 and 2758
I talked with my brother about it because he researched the shay locomotives a little bit. He told me that a family friend who was a history teacher in Dolgeville (who was also a local history buff) interviewed a few people who had worked for the Jerseyfield Lumber co. and they told him that the company only had 2 locomotives. My brother said if his memory served him correctly that the one loco was 90 ton and the other 70 ton, and that the family friend had pictures of both.
My brother also stated that the jerseyfield lumber co had a parent or sister company Goodyear lumber co in PA.
Also, there were several lawsuits against Jerseyfield lumber company - one from a company in western NY and a major one with the Brooklyn Cooperage co(who was Jerseyfield lumber co primary mill) it may be possible these companies may have obtained the locos.