clarence said

That’s why it’s a lucky break for shooters that ANY sight with the correct hole spacing can be used!  I’ve always wondered why Lyman saw fit to use a special base (which looks like the same one used for the ’86, though I’ve never compared the two side by side) for the SS model.  In the case of the ’86, moving the stem attachment back may have been necessary to avoid its being struck by the long bolt-throw of the ’86.  But what was the reason it was thought needed for the SS model?  

I suspect that it was to facilitate easier loading of the cartridge and clearing of the empty.

Bert