EV,
Close, but I just measured and it is 4" X 3.75" and that notch is about 3/4". All but one side is very roughly cut, so it seems like a rather quick & dirty job. I'll have to try reinserting it in the well to figure out the proper/original orientation. I suspect the notched corner fits in the top-aft corner where the glass is rounded by the fillet.
On the bolt, yes, my trunk had the 5/8" pivot bolt low on the trunk, then the approx. 3/8" lock bolt hole near the top toward the bow end, then this block's 3/8ish" bolt toward the top toward the trunk's aft end. Both the lock and block holes are well above the waterline.
If you scan my few posts by going to my profile link, I shared my trials to free up and remove the keel in an earlier post. I thought that block's bolt was the pivot bolt. I later learned the factory pivot bolt must have been leaking and the P.O. must have covered it with plastic medicine dispensing cups and a ton of silicone caulk. The keel came out much more easily after finding and removing that one!
The keel was bound in the lower part of the trunk due to the 4"X 5/8" steel core bar having rusted. It had expanded to the point of cracking the fiberglass filler and outer cloth laminate. There were also cracks in the filler where the upper flat section flair out to to the cast lead bulb section. The steel core appears to have been cast in the lead bulb. As you note, there were two styles of keel in these boats. Mine was supposedly the older 'composite' style. Given the delamination issues and apparently multi-step construction process, I can see why they went to cast iron, not to mention the probable health issues of working with the lead.
I am now working on rebuilding the keel by cleaning that rusted steel core, reconnecting the old glass/filler piece to use as a crude plug for laying up a glass mold, and then I will recast the steel core/lead bulb in polyester or epoxy filler so we can shape it close to the original, glass it all with a few layers of fiberglass and put a final coating of paint on it. Phew!
We'll see if anyone else has blocks like this...
Ed