Hello @OGMuffintop thank you for getting in touch with us and sorry to hear you have been unable to obtain the Curved Grip from the Iron Horse raid bosses. This particular issue has not been reported to us so far that I can see.
Is there any chance you can capture a video on your PS4, where you are completing the end of the raid, then show your inventory with the Curved grip missing but all other parts available? can I also make sure you are completing this raid on normal difficulty as well?
Playstation video capturing >
Please press your controller's Share button, this will allow you to capture a video clip, and then your console will ask you to upload it straight to YouTube or Twitter. After uploading to one of these websites, you should be able to post the URL link to the video in this thread.
In the meantime, I've reached out to our Division team for more info on the drop rate of these parts.