OK, I just got a reply from the National Postal Museum and here is what they said:
From: Piazza, Daniel
Sent: Thursday, March 15, 2012 7:38 AM
To: 'whitey-x@att.net'
Subject: RE: Contact Us - General Question
Hi Jim,
Your message was referred to me for a reply. I am not aware of any restriction on owning such a die so long as you do not use it to produce anything. The Mercantile Corporation of Dayton, Ohio did indeed produce postal stationery after the turn of the twentieth century. Feel free to send the jpg and I will see if there is anything else I can tell you. Are you interested in possibly donating the item to the postal museum?
Dan Piazza
_________________________________________________
Daniel A. Piazza
Curator of Philately
Smithsonian National Postal Museum
MRC 570 POB 37012
Washington, D.C. 20013-7012
202-633-4737 (voice)
202-633-9393 (fax)
piazzad@si.edu
Fire & Ice: Hindenburg and Titanic
http://postalmuseum.si.edu/fireandiceMarch 22, 2012 to January 6, 2014
So on that note does anyone want to take a stabe at what this might be worth? I'm considering donating it to the museum, depends...
Doc