It seems that this "Cahill" tintype has been circulating for a while. It is mentioned in this archived net54 thread from 2-24-2005, see posts #1 and #7. It apparently was sold by 19thcenturyonly for about $2200. I wonder what path it followed before it got to baseballglovecollector.com and then to SCP.
http://www.net54baseball.com/showthread.php?t=76245