Colin, I took a few photos of the well and the 'plug' that fits in the bottom. Ignore the grp and stainless tube at the rear (my later mod for another purpose).
An old standard shaft 7.5 hp merc was originally supplied when I bought Elsa in 1982. This, when lifted, would sit with the o/b skeg in the groove and would hold the plug in place so the hole was faired off for racing. Only a small engine would fit in the hole when down. Modern 4 strokes of 8hp and above won't fit.
I have re-engined with an inboard diesel some years ago but retained the outboard well so I can access the prop from inside the boat .. hence the sightly modified 'plug'.
I had to hurry with the photos but you should be able to access them via this link
https://skydrive.live.co...7&resid=99995E4059FA9E17 !2734&parid=99995E4059FA9E17!238&authkey=!AO2uYDoZ62uXB5k
(you will have to copy and paste into your browser)
If that doesn't work I will email them to you. I'm not sure how they can be posted here directly.
There is a load of stuff about keels and outboards if you search old records in the forum and as Geoff Harwood has said there is much in the 26 technical handbook.
Chris.