Jeff - really interesting that you have had this hypothesis and have done this research. I can add something but not sure how much it helps.
In 1980 my mother wrote a bunch of HOFers and asked them to send me their autograph for my upcoming 15th Birthday and I got a respsonse from Rube. See photos. He mailed it April 17th and passed away June 1st.
All these years went by and it took SGC to point out to me that the signature on the card was a stamp. You'll note it is the exact 'signature' that is on his HOF card. I did a perfunctory search and couldn't find any other stamps. So that's kind of interesting to me in and of itself.
The paper that says Happy Birthday, seems to have Rube's actual signature. This shows that on this particular date, approximately 6 weeks before passing away, it seems that Rube signed a note from a fan, along with including two stamped items. FYI, my mother mailed out her requests about a week prior to the postmark, so the turnaround time was short.
I found Rube's obituary where it says he died, "after a long illness" as well as a quote from his granddaughter who said, "her grandfather had been receiving round-the-clock nursing care for several months. Mr. Marquard, she said, had cancer and was operated on two years ago."
https://groups.google.com/forum/#!to...es/Bm5mXwhInhM
I reviewed this thread on my phone while traveling a few days ago and didn't review again now, so I don't remember if there were any later examples of his signature - but I wanted to post what I had.
For the record, I wrote Rube a thank you note - after receiving his mail on May 6th. I felt good as a kid taking the time to thank him but of course felt terrible to find out he passed away so soon after.
Good convo all. Thanks again Jeff!