Rancho,
That is an unusually nice '99. You did good!
The carbine tells a story. It took the Army about 35 years to realize that cavalry charges were a thing of the past and that most cav fighting since the middle of the CW had been done dismounted. So they finally lost the "saddle ring" on the '99 carbines. The lanyard attachment was replaced by a scabbard, the trooper could draw the weapon as he dismounted.. The hump on the handguard is so the rear sight doesn't snag in the scabbard. Earlier carbines got the later updates, mostly. Some of them got sling swivels and a rifle-style handguard - supposedly they were issued to engineer and artillery units.
jn