I suspect that at one time all three of the pulls at the neck were the same. One ring was either lost or removed, a Layard was probably attached to the bottom one for the cap/measure and the other two for the strap or another measure, one of them looks polished or cleaned. Since it has a large carved ring at the bottom of the tip which could have been used for the strap I'm thinking the pulls were added later but well positioned because they seem to sit right in the center of the diamonds carved in the neck. The pull in the butt looks to be the same as the other three, so they could of all been added at the same time or are original to the horn. In the bottom Pic it looks like there is a groove where a cap may have been fitted so it could be twisted into place and not lost, got a better Pic of that, it must have been large based on the size of the carved ring. As for the carved butt, it may be a replacement, there doesn't seem to be much ware where the strap would have rubbed on it. The tacks in the cap are replacements, you can tell by the ghost ring left by the originals, which also makes ne think the butt is a replacement.
Not much help, just my 2CW.
Tim