I'm having a similar problem. I have found a bit of information online that indicates the most obvious option, or at least the option most of the early 81's were drilled and tapped for, was the Weaver #2 side mount (here is a link to it on Midway USA: http://www.midwayusa.com/viewprodut/...tnumber=119028). I am having a problem confirming the exact specs however. I have attached some photos of my model 81 for you to review. Mine is D&T'd for only 2 wholes at exactly 3 1/8" center to center. I have been told this is the measurement of the outside wholes on the Weaver #2 side mount. When I do get the specs for the #2 I will post them for you so you can check against yours. Good luck and let me know if you figure it out.