Looking at your picture... That broken side plate you have there is in fact the older version being p/n 106990000. Midway has those in stock. It might be worth your time to call S&W as they may have the updated version on stock p/n 108600000. however the old one will do fine in a pinch.

BLUEDOT37 is spot on...Replacing it is fairly straightforward. First of all do not pull the sear pin. Use an appropriately sized punch to keep the hammer and levers aligned as you push out the old sideplate/hammer pin from right to left. Then use the new sideplate/hammer pin to push out the punch back out. If you don't have a punch, a piece is straight rod or fat wire will do like a chunk of coat hanger wire for example or the butt end of a small drill bitt.

Lastly you need to snap the two prongs over the end of the sear pin taking care to not bend them. Look closely and you'll see the sear pin is slightly beveled on the and below that is a turned groove to receive those prongs. Support the pin flush with the right side of the frame on something solid such that it protrudes from the left side of the frame. Then, close to the pin head press down on the prongs gently until they spread slightly and snap into the groove. I've done this with my thumbnail before in absence of a tool however a small block of hard wood seems to work best although I often use whatever is close at hand the wood handle on a chip brush or a chunk of plastic. As I recall, the armorers tool kit includes a small hardwood wedge for this purpose among others. Good luck and let us know how it turns out. BTW, I'd not recommend shooting the thing until you get the sideplate replaced. You'll have problems with the slide stop either engaging randomly or not at all.

Cheers
Bill

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Because they don't make a .46