PGFT 188.8.131.52 has been released on 04-12-2012 and posted for the first time by marcoqc in my forum at that date. Credits to him.
You can read there the changelog.
I can guess it doesn't unlock maps for new protected devices
that appeared around January 2014
Actually protected firmware devices are Nuvi55/56/65/66/25x9/26x9,zūmo590,GPSMAP64,Edge1000.