Be sure to check out Vic Thomas site here for some great info and pics about Mosin Snipers--
Snipers of the Red Star
Okay here is a little history on the ex-snipers.
The most accurate rifles were chosen at the factories for sniper conversion. When built in the factory the Mosin snipers were drilled and tapped for the scope mount, and a bent bolt added. The most common version is the PU
Although there are a couple of other earlier mounts used including a PE
After the war, for some reason unknown yet, the snipers were converted to ex-sniper status (basically turning them into a regular infantry rifle). They converted the bolt handle to a straight handle, then plugged and welded in the holes for the scope mounts. A mosin in this condition is what is referred to as an ex-sniper.
They can be converted back again to sniper status, by drilling out/tapping the welds in the holes for the mount again (ranges from easy to incredibly hard), and rebending the bolt handle. Repro scopes and mounts are availabe all over the place for around $200-250.
To tell if yours is an ex-sniper there are a couple of easy ways to tell. First is on the receiver.
If you have a Tula built Mosin, it will have a CH stamped on the reciever near the barrel to indicate sniper-
Izhevsk rifles have a scope number stamped into the side of the barrel (usually crossed out during the ex-sniper rebuild)
If your ex-sniper is a PU sniper (Way more common than others), you will be able to see the 4 holes from the scope mount on the inside of the reciever (move the bolt back)
If it was a PE you will not see those holes, as you can see the scope was mounted to the receiver. If its a PEM you will see one (possibly two) holes more centrally located on the inside of the receiver-