The original title, "Hellblock 666", was changed to Hellblock 13 because of the heavily Christian area the movie was being shot in, this concerned producers who were trying to secure locations. They ran into trouble finding locations with the original title and were able to secure a closed down jail after altering the suffix of the title from '666' to '13'.
As is the case with many Troma Films, this was filmed and copyrighted a few years prior to its release (1997).