Well, the Jamerson half of your namesake was a jazz player of some accomplishment (including on upright
), even if not everyone knows it.
I don't know the setting of the gig or exactly what's expected, but if these guys have any familiarity with your playing and chose you, it must be because they see something in your playing (well, that or no one else could do the gig
Be honest with people, but don't sell yourself short. You might have more jazz in you than you think. Maybe see if you can get a little practice time with the guys beforehand.
Here's to stepping outside your comfort zone, without a little bit of which, no progress can be made.