Mine is older I got in the late 70's the model is Marlin 45-70 Guide Gun 18" Model 1895G....I had gotten it cause my friend and I were planning a trip to Montana to go Grizzly hunting, my sidearm was and still have it a S&W Stainless steel 357mad.....we never went.....
Yes, it has a 26 in. Octagon barrel. I'm a little frustrated with it because I purchased an aftermarket sight for it and while the rifle is quite accurate, I can't get the sights the way I want.
What I bought is a "milled from bar stock" buckhorn rear sight that flips up to be a "ladder sight" for long rang shooting. I've gone through several different height front sights, and have been unable to calibrate it to hit point of aim at 200 with the ladder at it's lowest position and hit point of aim at 100 with the sight folded down to the buckhorn.
I love the way the new sight looks, and I really hope I can get it to work. If not I'll go back to one like the original.