William's Gunsite makes a 'side mount' (part number SM-70) that installs with three screws (as opposed to the originals that use two screws and two pins). They are available from Williams, Brownell's & Midway for about $50.00. I have one on back-order right now.
You will have to buy a special set of ring from Williams too:
Expect to end up paying around $90 for both. When my mount arrives, I will document the proceedure for installing it (Williams charges $90 to mount it, but unless you live in MI. that may not be an option...)
In the mean time, I wonder if you all can do me a favor: My mosin appear to have two pin holes, and two screw holes in the side of the receiver that have been 'filled' (you may not be able to make out the pin holes in this pic):
I want to belive that I have a factory re-worked sniper model, but I have seen this on other 91/30's so I am not sure - do your's have these features?