So, I think I figured it out.
The previous owner altered the stock sway bar mounts via a custom sway bar relocation (cut the bottom of the OEM mounts off). He then crudely fabricated some steel brackets to allow the stock sway bar to be bolted back in.
As such, my two options now are:
Go with the Whiteline Adjustable rear sway bar
Find a machine shop willing to custom fab a replacement bracket or two.
2011 Mustang GT M6 3.31s
Matt HONEYCUTT Tune, Lethal Off-Road X, GT500 axlebacks, J&M LCAs, UPR UCA, Whiteline LCA relocation brackets, Eibach Pro-Kit Springs, Koni STR.T shocks/struts, GT500 strut mounts, SR Strut Tower Brace
Best 1/4 E/T (05/18/14 MIR - no tune, X-pipe, or UCA): 12.920 @ 109.40 mph (2.059 60ft)