Rod,
Thanks so much for the info. First I have to state that I am not an expert on saddles. In Fact I noticed I put the wrong year for the saddle in the first place. I deal with 1858 Remington Revolvers alot and meant to say 1859 McClellan. Ooops. I would highly recommend the book you mentioned. I just got my copy this week. I had one on loan for about 3 months last year and it is really really intersting. I got mine for $66 new on Amazon.
From a glance through the book, you are correct there was an 1859 and an 1864 model of McClellan. The Civil war era trees were recycled until 1896. As you know the McClellan saddle went through several models. I think it was with the 1904 that the tree changed. I will be very honest that I am just not sure what the changes are. I will read more and can offer a more definative answer soon.
It is a common CW reenacting practice to use 1904 trees to make look alike 1859s. What we do is horse drawn artillery. We need lots of saddles, 6 per gun is regulation, and we needed that many at least. I bought 2 Grimsley saddles reproductions for $1000 each, but I certainly can't afford 6/8 of them. They also proved to be very uncomfortable, my drivers refuses to use them, so I bought 7 1904s off Ebay ranging from $150-$250. Out of all of those only one had rotten stiching so I removed the leather to make a cavalry (rather than drivers) saddle. Now from the book, American military saddles there are references to McClellan drivers saddles, but I am not aware of a surviving original, so leaving the leather on the seat, is a stretch as well. What I did was dye the saddle black, add skirts, change stirrups, and chnge the girth arrangement. I also added staples to the couple of non artillery saddles to start with. So admitadely what I have done is not 100% accurate, but I think it is reasonable. I do know the Confederates used regular jennifer saddles (very close to the McClellans) with staples and different stirrups as artillery saddles.
Doug Kidd's web site, Border States leatherworks has pictures of all of the variations of McClellan saddles.
Now I have proved how little I know.
Thanks again,
Steve