////Never heard it called bass violin before. Double bass, contra bass, bass fiddle , string bass, bass viol but never bass violin.////

It's also called a bull fiddle and a doghouse bass and, yes, a bass violin.

However, a bass viol is a different instrument. The viol has 6 strings and the neck is fretted for precise intonation. The frets were made of strips of gut or rawhide wrapped around the neck. They are wetted, tied around the neck at the precise spot and then dried in the sun. As the strips dry, they shrink until they are bound so tightly around the neck that they will never move.

The viol is much quieter than a double bass. Double basses boom but the viol is meant for a more intimate setting. They are not built at all like violins internally and are actually lined inside with linen to deaden the sound. They have c-holes instead of f-holes (although you might come across an occasional f-holed viol).

Double basses and viols are both tuned in 4ths. That's about all they have in common. Viols also have flat backs. Cheaper bass violins have flat backs but most have carved backs. Gasparo de Salo, the great luthier and bassist, did make his first bass violins with the bodies of viols but they were internally constructed like violins. Not sure how many strings they had as 3, 4 and 5-string basses were common back then as were all sorts of tunings.