I'm not certain what type of adhesives Winchester used, but I have only seen one loose forearm not on a 1885 that the insert was missing from. I don't think Winchester inserts were horn from what I've seen looking at those still on guns. I've seen scuffs on them and that would be tough to happen to horn. Horn is much harder than ebony, and a lot tougher to file down and shape. It's also brittle, and I've had to repair a number of horn tips on Ballard deluxe rifles. Not just because they're horn, but because they're so thin and delicate out on the ends.
I use two part epoxy to hold inserts in and also use it to repair damaged horn tips. I make a dam around the broken/missing piece and pour the epoxy in. Once cured I remove the dam and shape the epoxy down to match the rest of the tip. I use epoxy dye to color the epoxy as close as possible before pouring it, and then touch it up after finishing if it's not a perfect match.
This is an old repair I did to my 4 1/2 A1 Extra Ballard right after I purchased it years ago:
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Ebony as used on a Winchester stays black, but horn over many years turns a brown color over time.

calledflyer wrote Today at 11:17am:


Winchester grip caps were hard rubber, not ebony or horn. Grips with the vee insert were the same as their forearms. 
Horn is much harder than ebony, and it is so hard it squeals as I run the file across it when shaping it down! I cut the horn inlay down with my saw this morning and then began filing it down flush. It squealed with every stroke of my mill file as I worked it down close. Have to be careful sanding it also as I needed a hard backer to the sandpaper to avoid taking the surrounding wood down faster than the hard horn insert.