I believe the U. S. Cavalry left the sight guards on all the time.

The device helped avoid damage to the leather scabbard, as well as, damage to the sight-blade.

The 'hood', of the sight-guard, was large enough to allow the carbine to be sighted and fired with the guard in place.

By contrast, the similar sight-guards, used on 1903 and 1903-A3 Springfield Rifles, had to be taken off to have a useful sight-picture, when shooting. These 1903 protectors appear to have designed for shipping and storage.

p.s. - Workmanship of Butt-Swivel installation, on OP's carbine, appears a bit cruder than I would expect, if installed in 'The Field' by an Armorer with an Armory supplied "Kit". (Just my ignorant guess & opinion).