Funny, my '73 Special doesn't have this cutout. Just a very regular formed 'tunnel' for the exhaust pipe.
Lotuses..... every one a little bit different, don't ya just love 'em ?
As I said Peter, I've no idea if it's OEM or not, it's always been there and I've had the car for 25yrs of it's 40yr life, with only one other owner. Apart from adding cosmic alloys he didn't even fit speakers to the door panels (the radio was loose in the glove box with a horrible boxed speaker above it) hence me thinking he didn't do it.
Maybe it was put in when the stainless exhaust box was fitted as that's clearly a one-off thing, or perhaps the two drain holes blocked and collected a couple of gallons of water in the rear valence and this was a solution, heck I can't even make a sensible guess.
But it is very neat, with perfectly rounded corners and properly finished off.
My last guess - I wonder if the exhausts burned the paint on the first TC's, the quick fix was "cut a hole in it" followed later by "alter the molding to give a bit more clearance like a proper designer would do" ? It seems Chapman-esque enough to be real, although I bet whoever added more fibreglass later got the sack !