Lee included a .264" and a .268" so you could shoot either .264" or .268" bullets ... simple as that.
My collection of shooter, 6.5 X 52 Italian Carcano rifles consists of a M-1891 Rifle, M-1941 Rifle, M-1938 Short Rifle, M-1891/38 TS Carbine, M-1891/38 Moschetto Carbine. Some will shoot fine with a .264" bullet, others require a larger one.
In all instances, I use only Hornady 160gr RNSP, either their .264" or .268" bullets. (Which I modify them by running them through a Lee sizing die and squeeze them down to .2665" because when I miked one of my surplus rounds, I read the bullet at .2665, not .2680").
I like this bullet because it is HEAVY ... it is LONG ... it has a LONG BEARING SURFACE ... it is FLAT BASED.
I use Hornady's recommendation of H-414 powder and (Winchester) Large Rifle Magnum primers. My load data calls for 34.1gr and I seat the bullet to give me a LOA of 2.985". It's a gentle load since I just shoot paper.
This is the loading data I use and while safe for my rifles, it may not be safe in yours ... use at your own risk (legal stuff).
Unless you are willing to remove your bolt and clip a single round under the extractor and them reinsert the bolt with each shot ... you will need a clip. If you try to slam a round into the chamber without using a clip you run the risk of breaking your extractor. (The bolt removal option is not as big a pain in the butt as it sounds. Open the bolt, pull it to the rear, pull the trigger and while holding the trigger back, pull the bolt to the rear and out. Reinsert the opposite way. This will slow your rate of fire down and will keep your barrel cooler and cause less mirage so your sight picture doesn't waver all over the place).
Speaking of sight picture ... unlike as we have been taught, Italian practice was to bury the front sight blade in the BOTTOM of the rear sight "V" ... and then I aim at the waist of a silhouette target with a Shoot N C in the chest area. This seems to work for me.
Hope this helps. I'm a certified Italian Milsurp nut and the M-1891/38TS I'm shooting is one my Dad sent back during WWII. It was my very first Milsurp and it's what got me into reloading ... but that's another story for another day.