Yes, I read the whole thing - couldn't put it down.
It's this. I can't remember who recommended it to me, but thanks.
Really, this was an aspect of the Franco period that I never knew about. Also, the author goes into the whole notion of secret Jews in surprising places.
These are families that have outwardly embraced Christianity for generations, but either still preserve the secretive knowledge of their origins, or, even odder, maintained certain rituals that can be directly traced back to Jewish practice, but have been altered over the years in subtle ways.
One thing common in many of these families is the lingering fear of discovery, and, in one chilling case, that fear was warranted. Apparently, in the 1920's, a number of families in the Netherlands were persuaded to come out and declare their Jewish roots. That worked great until the Nazis marched in 20 years later, rounded them up and killed them.
The families that did not reveal their Jewish past survived.
Once again, it proves the old adage that "I wouldn't be paranoid if They weren't out to get me."