Question on Front Turn Indicators

My '79 308 GTS came without front turn indicators installed. However, there was also a cardboard box of parts that included some very raggedy front indicator housings and equally raggedy "JB-welded" lenses.

So I got me a pair of new turn indicators. On delivery, I discovered that in order to get the new indicator housings to fit in the existing front bumper, they have to be modified (two locations, described in detail below). Looking at the "original" turn indicator housings from my parts box, the same modification had been done on them.

What is going on? Is it the wrong front bumper? Are there different turn indicator housings?

Details:
Here's the opening in the bumper for the indicator housings. Note that there is no "notch" on the upper part of the opening (like there is on the bottom) and the retention slot on the side is relatively shallow
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Here is the new turn indicator. The retention feature on the side is too long and the notch alignment feature is on both the top and bottom of the housing.
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And for reference, here's the "original" turn indicator housing. You'll note that it's been modified to "fix" all those fitment issues. Also note, this is a relatively old turn indicator housing. It may not be original to the car (as there's no way the factory was grinding off features), but it's definitely been installed in the car for a reasonably long time (I'd guess it's at least 30 years old based on both the type/condition of the PVC jacketing on the wiring as well as the oxidation level on the ground strap & retaining bolt).
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No grille (one of many items on a looooooooooooong list), but here's a shot of the entire front bumper.
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Yeah, thanks for understanding my actual question @BrockBenson (why, not how).

The indicators are definitely reproductions. I got mine from FerrParts, but they look identical to the replacement parts sold by Ricambi, AWItalian and the various reputable dealers on EBay. They are produced in Italy by Carello (an old lighting and lens mfg--no idea if they were the original supplier, but it would make sense). I was trying to understand if a) all new reproductions simply need to be modified for the 308's, b) I received/purchased the wrong reproduction part or c) the bumper ain't correct. I would assume for anyone who's had to replace turn indicator housings, this would be an already known answer.

As a point of interest, those "original" housings (or, at least the ones that were in the parts box and look to be ~30 years old) are also produced by Carello--and as mentioned before, had also undergone the same modification that I'll need to do to these new ones in order to make them fit.

My suspicion is that the housing itself is made to be "reversible"--can be put on either side. The only thing that changes is the lens cover itself (left vs right). A small piece of stamped metal also has to switch position. From a mfg perspective, this would make sense--keeps part count low. That would explain the "notch" being on both sides--depending on which side it's installed, the top "notch" needs to be ground off. On the retention feature, my guess is this part fits on a number of 70's & 80's generation Italian makes/models and some of them require a longer feature. Again, makes it simpler to mfg a single part that has to be modified slightly depending on the vehicle.

But again, I was hoping to get some clarity from those that know