The Williams FoolProof AG(high) or GR(low) TK (the 'T'arget 'K'nob version) is designed as a 'universal' rear sight to clamp onto 3/8" rails on any gun so cut. (As everyone seems to be interpreting the rule, that makes that sight 'made for the rifle'...no matter what rifle you have. )
You can see the description by going to the Williams website and downloading the catalog. It's on P 11 -- right column.
http://www.williamsgunsight.com/PDF/2004catalog.pdfThere are other FPs -- and Redfields, Lymans, etc -- that screw into the side holes and are designated for a specific model gun (or models).
You could equally well use the FP 39 TK screwed into the side holes.
As I understand it, Brownells always seems to have the slip-on model in stock with the target knobs but the side-mount screw-on model with the knobs is less frequently available.
So I'm seeing a lot of slip-ons and no screw-ons. It's not really significant as long as you get the knobs.
As far as I'm concerned, I'd rather have the Lyman with target knobs since IMO the Lyman version is marked better and more 'readable' but, last I spoke with Lyman, they were only offering the model for the Winchester 86
http://www.lymanproducts.com/lymanproducts/sights.htm set up that way. I did see at least one rifle -- that was NOT an 86 Winchester -- with that target-knob sight on it at nationals. It had passed tech and was not protested...
AFAIK the screw pattern and spacing on all flat-sided levers by all the makers -- Winchester, Marlin, whatever -- are the same and the differences between the sights actually 'designed' for those models are very minor. IOW, I'd guess that a Lyman 66WB will screw right onto a lot of rifles that it is NOT marked as being made for...
This is another rule that tries to cover a lot of ground and leaves a chunk of wiggle room. I just wonder how much importance we need assign to whether the sight is actually 'made for the rifle'?
As long as full-house Warners with Gehmann diopters don't start appearing on 'cowboy' guns, methinks it ain't all that big a deal.