Hey Dark,
I got a Marlin 1895 Cowboy a few months ago. Other than the lousy Marbles rear sight it came with, I think it is a beautiful piece from the old west. With my original rear sight at its highest elevation, I was still shooting a foot low at a hundred yards. Then the screw in the rear sight leaf plate fell out. I called Marlin's customer relations and they promptly sent me a new Marbles buckhorn sight. I still had the same problem with elevation, so I installed a Lyman #2 Tang sight, removed the Marbles rear sight, and replaced the rear sight with a Lyman folding leaf sight.
The Lyman tang sight for the 1895 comes with a replacement tang screw and an elevation plate. It does require one hole to be drilled and tapped in the tang, but it is an easy job to do. My tang sight required absolutely no shimming to insure it was plumb with the barrel. The Lyman sight has enough elevation to reach about 500 yards I would guess. I have only been able to shoot as far as my rear pasture which is 300 yards. There is no windage adjustment on the Lyman tang sight, but you can drift the front sight one way or the other if necessary. I found that I did not have to move my front sight at all once my tang sight was installed.
It also comes with two different sized aperatures: a small one for long range and a larger one for hunting purposes. Some folks say they remove the aperature insert and use the tang opening as a ghost sight. For me, that is too large a hole for my old eyes. You could also install a Lyman receiver sight without drilling any holes. The receiver is already drilled and tapped for a slap on receiver sight. In my opinion, however, the Cowboy gun looks better with a tang sight.
Marlin has a forum where you can search a lot of this information. As you might expect, most of the folks who use the forum tend to be Marlin oriented, but it is a pretty good forum.
I have my folding rear leaf sight set for a 100 yard point of aim. Then I have my tang sight set for 300 yards. If I use the tang sight, the rear sight gets folded down. You really cannot leave the buckhorn sight on the barrel if you install a tang sight because it gets in the way of the tang sight aperature picture.
A Lyman tang sight is somewhere in the $60 range, and a Marbles tang sight is about twice that amount. The Marbles does have windage adjustment, but it just does not seem worth the money to me. If you install a Lyman and have to make a windage adjustment, you drift the front sight once and let it stay there. The only adjustment I now make to my tang sight is for elevation, and the Lyman does that quite well. If using the folding rear leaf sight, the Lyman tang sight folds to the rear and is out of the way.
If you are shooting a load with heavy recoil, the tang sight can poke you in the eye if you position your eye too close to the sight. I do not want to make you unduly concerned, but you do need to keep that in mind when using a tang sight on a rifle with heavy recoil. That same thing is true with a scope hitting you in the eye if you creep up on it too closely. Just a warning reminder.
I like the cowboy, and I like my Lyman tang sight. It looks period correct for a Cowboy gun.
Best wishes,
Dave Wile