I know that the late-war 1911A1s were made with a notch on the upper edge of the rear portion of the trigger bow. Current triggers seem to be a mixed bag - some have the notch and some don't. Is it really all that important?
I know what he is referring to. Never really gave it any thought.
Besides staying true to the gun as built I would not worry about it. Unless that serves some function specific to that frame.
I think it's just a relief cut to ensure that the trigger bow cannot engage the sear legs. Shouldn't matter if all the parts are in spec and tolerances not horribly stacked.
Sometimes you'll see cutout on the bottom of the rear of the bow. That's to ensure that excessive over travel can't cause the bow to lift the sear spring. Did that once on accident.
Sometimes you'll see other cutouts on the back/sides of the bow just to reduce weight.
The rear of the trigger bow is contacted by the grip safety. When fitting the grip safety, it is a gunsmith trick to notch the rear right corner of the trigger bow. This will allow the grip safety leg to retain more metal. This is handy if you ever need to fit another trigger.
The factory notch in the pic serves no defined purpose as far as I know. Might be some oft of index or register for locating the bends during manufacture.