Interesting question...
I have (2) SP101's and GP100. Just mounted a reddot on my GP by using the 6" B-Square mount, which screws into where the rear sight was and attaches where the front sight dovetail is. The front sight attachment of the GP is different than the SP's, but if you get a chance to look at the B-Square GP100 mount, it might give you some ideas.
I believe Brownells sells blank Weaver-style bases, so you'd have to machine/modify it to mate to the profile of your SP. I don't have either in front of me, but I wonder if the SP has enough metal at the muzzle to be adapted to use the Redhawk/GP style of attaching a front sight. If so, although that would require some fancy filing or machining, you'd easily be able to adapt the standard B-Square mount. You could get the spare front-sight parts directly from Ruger (the spring, dovetail locking pin and front sight for when you don't use the sight). There's no extra pin holding the GP100 dovetail lock pin in, spring pressure pushes it back against the front sight. Neat design, just like many Ruger innovations.
One other thought, I think B-Square also made an older-style mount that gripped the barrel instead of replacing the sights. Maybe you want to check their website, or check on eBay or Auction Arms.
Old No7
