I have just acquired a Martin Sigma bass through a trade. I was wondering if anyone knows history on when this would have been made, there are no numbers visible on the outside of the bass. On the headstock there is a sidways M with SIGMA directly under it. This is as if you were holding the bass upright. There are metal covers over the bridge & neck pickup which looks to be a lipstick style. Bridge pickup is a humbucker also metal like the lipstick style pickup. It has 4 screws across the middle. On the metal place where the neck is attached on the back of the bass reads "Made in Japan." I will take the pickguard off later today and see what I can figure out inside the instrument. I have searched for a few hours online and have not been able to find history on this bass. Can anyone help?