Just an update. Thanks to all for the suggestions!
After grinding down the hammer and spur with my Dremel as was suggested, I now have enough clearance for either
my Sightron 4x32 on Warne low rings or
my Nikon Omega 3-9x40 on Warne medium rings.
Maybe we should all send a suggestion to H&R to make the hammer shorter in the future, and have a shorter replacement available.....