What Belongs Here?

The triangular piece is used as a wrench on the assembly line to hold the pipe that comes thru the firewall when they put the hard lines on.
Sorta like using 2 wrenches on a fuel line.

My car is the same way and there are no holes in my firewall.

You need to cut some black foam to fit and stuff it in the opening.
Year One has a whole firewall opening foam gasket set if you want to go that way.