Nearly all before 1988 have it huh? Just a bit of a stretch there.... I've looked at a lot of pre1988 cat 30s... Can't say that many had the problem... Mine certainly hasnt had an issue with it... Perhaps it's more of a problem in an area that routinely hauls boats for the winter... Idk
The "smiles" may have been patched over too. Beyond 1988 Catalina no longer placed wood in the keel stubs of these boats. The smiles on post 1988 boats happens significantly less than on the earlier models, even when dry stored.
This repair is a major one, I've done it on our old C-30. Our keel bolts were sooo rotted where they passed through the wet wood that they needed to be replaced. I used the "barrel bolt" technique which I would not advise anyone to do. Mars Keel and Broomfield both replace bolts with a special process where they literally melt the old ones out and the new ones in. It is a like new keel when you get it back.
Removing the wood is tedious at best and is much more easily done without the keel in place. The engine on our boat needed to be removed to get all the wood out. On some years I hear the wood did not go that far aft..
Some of these keels were bedded with a fiberglass putty and some with 5200 or similar. Both can leak once the wood gets wet and begins to compress. I think the biggest culprit for moisture ingress is bilge pump and float switch installs by those who had/have no clue that there is wood in the keel stub. Once the wood gets wet it gets softer and softer until the keel nuts compress the wood enough to cause a "smile". The smile will not always mean a keel bolt leak but very often can.
The easiest way to determine the extent of the problem is with some keel stub core samples.. If you find moisture it would be a wise idea to do a keel re-set and remove the wood core.
I had a smile repair this spring during my haulout on a '90 C30 Mk II. Maybe more of a grin than a smile...the crack was under 1/8" and about 6-7" in length both sides of keel.
Yours was likely due to improper blocking, a grounding or keel bolts that needed to be re-torqued. By 1990 there was no wood in the stubs..