Mosin markings are an adventure on their own. If you look at some of the milsurp and mosin nagant websites, you'll see that there are alot of arsenal marks that are still a mystery to collectors.
As to why your Mosin has older style marks, I'll venture a guess. If its a 91/30, then it was likely never built that way. 91/30's started production in 1930. Could be you've got an older rifle, a Cossack or Dragoon, that was cut down and modernised.
As to the old-style markings on an newer rifle, it could be that your rifle was made during a period of transistion from one set of markings to another, and there was some overlap in between.
I'd recomend that you take a trip to websites like surplusrifle.com, 7.62x54.net and some of the other Mosin-Nagant collector websites. It could be that you've got a valuable rifle.
If you can, post up some pics of the rifle, and its arsenal marks. That might shed some light on the mystery.