The first time I remember seeing a 5-string upright bass was Harvey Weston with the Alex Welsh band around 1969. He also played 5-string bass guitar. I remember him lending his upright to the bass player in a support band whose bridge had collapsed and said player had clearly never seen a 5-string upright before. When did these come in? Presumably, there was a musical advantage, although classical players manage with 4. In the ensuing years, more and more jazz and blues players have 5 string upright and particularly, bass guitarists, seem to play one, and often also have a 4-string for some numbers.
I ask as a naive non-musical washboard & kazoo player so don't waste time flaming me for my ignorance.

RtS