I've been down the Krag rabbit hole a few times regarding receiver sights.
Here are a couple observations. The rifles, for me anyway, didn't feed worth a darn without the cutoff spindle in place. That spindle is pretty necessary to act as a cartridge guide for the round as it loops its way up and around the side of the magazine. Some of the "no-drill" sights incorporate a dummy spindle that acts as both a locator/base for the sight and as a cartridge guide. The most common of those back in the day was the Pacific sight. (A very simple sight that worked/works very well, but has become kinda scarce and no longer very cheap. Mine served admirably for quite a bit of hunting and target shooting. I have one waiting its turn on a future project.)
The other most commonly found no-drill sight is the Redfield. I think all Redfield Krag sights were no-drill types but don't quote me. They, or at least the couple I messed with didn't incorporate a a fake cutoff spindle and had to have a separate fake spindle inserted to ensure reliable feeding. Minor PIA. I epoxied in a spindle I cut off of a spare cutoff (pun intended) and it held up well for as long as I had it- but no guarantees as to long term viability. I've heard of guys soldering instead. I've also heard of guys not having issues running the gun without the spindle too, so what can I say?
The Lyman 57 Krag sight is another beast entirely. It gets screwed to the RH side of the receiver conventionally. (Oh boy, we get to drill and tap another harder than woodpecker lips Krag receiver!)
The Cadillac of them all though is the Lyman 48 which goes on the LH side but requires d/t'ing. Worth the expense (when you can find one) and the hassle of installing, but boy it works the best and just simply looks the coolest! I'm installing one on my current Krag project (that's been hanging fire for waaaay too long now).
There is another Krag sight that pops up now and then called the Rice sight. It was/is a simple little stamped sheet metal affair that attaches to the top of the bolt tail block, and has no screw-in aperture just a peep hole. I had one once that actually worked quite well and didn't require the horizontal sight arm detachment for bolt removal like all the rest do. They sold originally for a buck or two, but one's I've seen recently carried collector's prices.
Sorry for the diatribe, but I'm half wired from a bout of insomnia.