I'm a bmw owner who got this rail/slider in a deal with a sparco seat. I have no use for it and would like to sell it:
They look exactly like what someone posted as 996/7 sliders:
I also posted on rennlist and one of the guys from OG racing confirmed what it is.
$110 shipped anywhere in the continental US.
Since I only have one post here you can check out my sn on bimmerforums (r0b0t) or nasioc (sti robot) or iwsti (sti robot). I have seller feedback on each of those and can confirm my identity via pm on those forums.