I bought some used brass which was supposed to all be 243. I found this case in the bag. It has 243 head stamp, and has been fired. Here it is in the center of photo. 243 on left, mystery case center, and 6 SLR on right. The shoulder is actually moved forward of the standard 243.

Any idea what it is? Looks like SLR but with short neck of 243.

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