What behind the saddle is this?

[DavinFelth]

See attached
.

[DoronG]

Shimano Pro Tri Cage Saddle Mount

https://www.amazon.com/SHIMANO-Pro-Cage-Saddle-Mount/dp/B00KDMW8VO/ref=cm_cr_arp_d_pdt_img_top?ie=UTF8.