Gelcoat blistering is common on many boats, rarely going past the layer of mat just inside the gelcoat.
Vinylester wasn't used in the 70's as far as I know. The company with the fire retardant resin additive was Uniflite, who also built the Valiant. The problems started when they had to change the fire retardant additive due to the lack of availability of the one they had used originally. This type of blistering was unique in that it concerned the entire hull structure, not just the mat/gelcoat exterior. I don't think any company except Uniflite used fire retardant additives, and certainly wouldn't have after Uniflite's well known problems. The original reason for its use was the US government contract for 31' gunboats for Vietnam, which required it.
Living aboard in Victoria Harbour