JT,
The one 500 pictured (above) is one of two types as he said.
While one could be fooled into thinking the one pictured above IS a dovetail type, it too is retained by pin(s). The pins pass through the barrel shroud on the type pictured above, in such a position as to disallow the front blade from working it's way up and out. The pins go through at the narrowest point of the lower (mounting) half of the sight.
The other type (I describe this style of mount as a biscuit or woodruff key) is the second type 500 described. This is the one that must be fitted and drilled for the pin to pass through it. My J frames have the later type. If you decide to go Tritium, and there is NOT a direct replacement available (if the pictured type), I have seen the woodruff style blade, used in the trapezoid style milled guns, by using only one of the two pin holes available for mounting. But doing it that way, will leave a portion of the milling notch for the trapezoid style blade still visible upon completion. IIRC, brother member Headknocker has done this conversion and may still have the pictures floating around. Hopefully he'll read this and comment.
GREAT PIX as always 500!