This scarce example was made (and marked) by ESPERANZA S.A. They are well known for their telephone insulators and other high voltage glass power insulators in Spain. Measures 16 1/2" long x 10" wide.

Unfortunately there is a corner piece that broke off (seen in photo) and 2 fingernail size corner chips and a approx 2" slice. I've never seen another one of these before. Apparently used with a large battery and/or accumulator for electrical storage.