During that time the civil servants working at the registry had ample provision to decide what names would or wouldn't fly, and many made up their own rules. This led to quite different rules in different parts of Spain.

That being said, these were civil servants under a fascist catholic dictatorship, so you can imagine that the range went from "only catholic saint names in spanish" to "if it sounds spanish-enough to me I'll allow it". Foreign names (John) were not allowed in most cases, just like they didn't allow names in any of the other languages of spain such as Joan (catalan), Jon (vasque), Xoán (galician).