Not located on the Rostrum proper, it's on the main wall to the right of the West Base route. As you hike down the Rostrum descent gully, just before reaching the Kaukulator and rappels, this hanging flake will be on the right.

Start by underclinging a wide crack (not easy to protect) and then pull over into a gully. Ascend the gully to a big flake and undercling left again. Lieback the edge of the flake then traverse right to rap slings on a big flake/horn.

Awkward climbing, and the rock is sort of shit.

Pro to 4" plus I placed a tipped out #5 Camalot near the beginning (a #6 might be better). Bring some long runners to extend pro since the route zig-zags a lot.