Possible. That brace is often spaced down to allow anti lift geometry rear arm pivots. The spacer kits come with new hardware and sometimes they use generic bolts that don't follow automotive sizing. Surprisingly you may find they have a 5/8" imperial head on a metric bolt.

this certainly is a possibility

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Nope, the bolts/nuts are 17s (measured 17.2mm, i did rough it up a bit)

Whats interesting is that the 16mm socket has either been warn out or is incorrect because it is 17.4mm.

Socket number 17 is 17.24mm...smaller than 16 :0

Mystery solved.

Dt, you know your Subaru!