Grimsleys lack the oval cut-out over the horse's backbone that is the hallmark of the McClellan and which this saddle apparently has; therefore this isn't properly a Grimsley. Most likely, it is a non-regulation private purchase officer's saddle that combines features of both.
Okay--we are going to get into technical terms here.
The wooden
tree of both saddles has the cutout, right? The wooden tree can be either covered or uncovered, but the
cutout is always there in the wooden tree to protect the horse's spine, in both the McClellan, the Grimsley, and every modern
western and
English saddle.
The Civil War McClellan has a wooden tree with a cutout and a rawhide covering that shows the cutout. The name is based on the shape of the tree. Here's a
modern McClellan saddle, still called a McClellan, where the opening is covered but the name is the same, still based on the tree.
The Civil War Grimsley has a leather covering that conceals the cutout, but the cutout is still there in the wooden tree. If you took the leather off, the cutout would be visible. The name is based on the shape of the tree.
So the picture shows a Grimsley tree that's not covered across its cutout, though it does look like it has a nicer leather covering, better than rawhide. I still believe it's a Grimsley saddle. The name is based on the shape of the tree, rather than the covering.
For example, here's a Grimsley tree covered in rawhide like a Civil War McClellan. You can still see the raised "thingy" along the back.
https://new.liveauctioneers.com/item/11570169
I believe that the one in the OP photo is a non-standard Grimsley. It has a line that matches the usual brass trim around the edge of a Grimsley. I'm wondering if the center wore and was replaced with leather that fit the tree more closely, and therefore showed the opening, while the front and back still match a usual Grimsley with its brass trim.
Also, in looking at images, I noticed that
dragoon saddles, from a slightly earlier era, have that thingy rising in the back. Is the saddle in the picture possibly an old dragoon saddle?