I have a centre binnacle to fit to my mini, the one pictured below infact.

The problem is that I can't figure out how to fit it. Usually they have a lip in order to screw/bolt through and on to the bulkhead. You can just about make out on the pic that mine does not

Did the very early binnacles not have a lip or is there another way to fit it that I'm missing? This is a reproduced one just so you know.
Any help much appreciated.
Cheers
Chris