You might try the Natick historical society. It looks like they have a museum in the old factory.
http://www.milb.com/milb/info/museum..._massachusetts
The price for that letterhead amazes me. It's not even the full sheet. I had three of them at one point, two partials like the one on Ebay and one complete. The full page is closer to legal size.
I sold two of them I think, and didn't get anywhere near that price. I know one went for 100. I'd almost say the one from Ebay was one of mine, but I don't recall the embossed bit being embossed that well.
I guess I shouldn't have mentioned that the one I sold was a half sheet instead of a full sheet.
The one I have a scan of.
Steve B