I got the replacement piece. I have my own theory on why some are getting it. Here are a couple pics I took:


In-hand, they are essentially identical. There is only one major difference I could see (any other differences you see are on account of lighting and amateur photography). In the first picture, you see those two, uneven tabs coming out of the bottom of each piece? Those tabs connect the gun to the back of the trailer in vehicle mode. Now, in the second picture, see how the rightward tab on the bottom piece looks white on the end? That's the original - in-hand, it looks like like the tab is either broken or was manually shortened with an exacto knife. The one on the top is the replacement, and looks cleanly molded.
When I opened Diesel up and began to transform him, the very first thing you do is take off the gun piece from the back of the trailer. And the very first thing I noticed when I did that very first step was the 'damaged' tab. Just from detaching the gun from the trailer, you would think the tabs to plug it in should be of equal length. I thought mine was damaged out of the box, and I was mighty pissed.
Then, I saw the replacement, which has the short tab molded that way, and I figured it must have been intentional. Only once he was combined in gestalt mode did I see why they may have done what they did.
When you plug the gun into the lower arm, the cavity that holds the gun is sloping - that's the area where Last Chance's back windshield is. If the tabs were the same length, the one that's shortened would not fit well into that cavity due to that slope, and would pop up slightly.
They must have realized this late in the game, and fixed the issue manually. I suspect they then did a simple remold, and a new run of the part to include for some retailers because they knew people (i.e., me) would think the original piece was broken, or demand a new one due to the poor cosmetics. Who knows? Maybe that's why he got delayed from December to January.
Those whose Diesel doesn't come with the extra/replacement piece, see if the piece in the box looks like that short tab was cut manually, or if it looks molded.
Phew! That was entirely too much description for a single tab on a single piece.

Coffee time