My guess is was in the 60's when resin was impervious to water.
Something changed it was not until say the 2000's when monohulls
started using Vinlyester resin, or whatever to resist blistering.
If you want to keep you boat in the water, and want to prevent blistering,
I recommend 5 coats of VC17 bottom coat expoxy to seal the gelcoat.
I tried 1 coat not following manufature's instructions and it blistered again.