You have three questions there.
That battle sight on any No.4 or No.5 is superior to almost any other WW2 battle rifle. Results at my club's milsurp matches prove that. The only rifle more consistently at the top of the winners' list has been the Garand. The Mausers with forward notch sights just don't compete, and the Swiss rifles do very well except for the cost of their ammo. So, do you have a 90-deg flip back sight in which case yes better ones were issued? Or, is the sight something someone has modified (ie Bubba'd) along the way?
Secondly, 7.62x39 to .303BR is a known down grade to make them more affordable to shoot. Reprime fired Boxer primer cases and move the components over. I say Berdan not Boxer primer cases, because poking out a Boxer primer is just so much easier than trying to hook out a Berdan primer. There is no central flash hole, only a built in anvil. Commercial 303 is most likely Boxer, as well as Canadian wartime DI Z cases.
Third, I don't understand what you mean by clips. From you posting count, I think you are not long in the shooting game. Terminology is vital when discussing firearms. A magazine is not a clip. Even though the four-letter word gets applied to the seven-letter word incorrectly. A stripper clip holds 5 or 10 loose rounds. For many military rifles, there is already a slot in the top of the receiver so rounds in stripper clips can be top loaded into the rifle's attached magazine. Stripper clips were one-time use to top up a loaded magazine. Got it? The exception is a US Garand which has no box magazine, and the 8-round "en bloc" clip seats inside the rifle and the mechanism loads the ammunition from the clip until it is empty and you get that lovely "PING!" ejection.
