What you have sounds like an "original" mini 14. They all have side mounts for the scopes, which look slightly offcenter from the rest of the rifle. The reason for this side mount is that the original mini's ejected shells at a very steep vertical angle. The ranch rifles eject the shells at a very flat horizontal angle. If the scope was mounted directly center of the rifle, the brass usually hit the side of the scope. If you know someone that has a ranch rifle, the original will be slightly longer and will have the mounts that others mentioned. In my experience, my father's original mini-14 shoots MUCH better than all of my friends mini 14 ranch rifles. They did not start printing "ranch rifle" on the receivers until 2000, but ranch rifles were made before 2000, just not marked on the receivers. As far as I know and have read, ranch rifle seriel numbers start with 187 or higher. Here is a link on ruger's website to look at these seriels and see the years, etc. Hope this helps.
http://www.ruger.com/Firearms/PS-Ser...story-RI.html#
P.S. I did a quick search on different scope mounts and came across this.
http://www.gunaccessories.com/Milita...uger/index.asp . I don't know that these are the best prices but will give you some model numbers and ideas for mounts that might work for your rifle.