That duct is different than what most of us here in North America have/had in our cars. In our cars, that covered hole is open and it's what ultimately feeds the stock intake with air. And we don't have that square hole with the riveted flange that's just below the covered hole. My guess is that your duct is for a later series of the FD. And I'm not sure how those were set up from the factory.

 

 

My reference is my Series 6 (1993 model) and it's different than what you pictured. It does not have that flanged opening and there is some minor differences in the air guide (black plastic piece that sits on top of the duct). Again, I think your duct is for a later series.
Not from my car, but my duct looked like this.....



A very old photo of my car (note the different air guide)...