Alembic was making multi-string basses in the 70s, I have a 78 Series I five-string.
No idea how accurate the wiki entry below is but it claims Carl Thompson made the first bass with a B string for Anthony Jackson in 1975...
If you want to post pictures you need to upload them to a hosting service first... I am sure everyone would like to see your pics.
"In 1975, Anthony Jackson asked Carl Thompson to build him a six-string bass guitar tuned (from low to high) BEADGC, which he called the contrabass guitar. Jackson's bass extended the range of the bass both lower and higher than a four-string. Though Jackson initially received much criticism for the new instrument, the deep sounds of the low "B" string has become a standard in many genres including metal, R&B, funk, and gospel."