At some point S&W started machining some of their double column magazine 3rd gen full-size guns so the mag catches could be reversed. (I think it was later in production of the 5903/06 & 4003/06 TSW series?) The full-size 9/40 frames were machined with the small hole required for the mag catch nut's plunger & spring located on the right side of the frame (although "reversed" from it's orientation on the left side) and the same mag catch assembly could be switched from one side to the other (and adjusted, via plunger height, as needed).
The single column magazine 10/.45 frames, when machined to accept them, required a special "left handed" mag catch (which used with the existing nut, plunger & spring of the stock right-handed mag catch). For example, the current 4566TSW can be switched using the optional left-hand mag catch body, but the current compact 4513TSW I was issued isn't machined to accept it.
Naturally, the magazines used in guns with "left-handed" oriented mag catches required the ambi cuts so they could be used in the guns with the mag catches on the right side of the grip frame.
I found it a bit interesting that when we started receiving new TSW's that either had the ability to switch the mag catches over to the right side, or could be used with left-handed catches, none of our left-handed shooters appeared interested in having their duty guns modified to "left-handed" mag catches. Apparently they'd become accustomed to using their "right-handed" oriented catches.
