Joe, got this info comes from the Bass Book by Bacon and Moorehouse:
What are the differences between a 4001 and a 4003 bass?
Here is a history of 4001 and 4003 production, composed of information from the sources given below:
4001 - '61 to '80 (or possibly as late as '81)
4001S - '61 to '69, '80 to '84
4003 (1st version) - '80 (or '81) to '84 truss-rod adjuster at body end two-piece pickguard
4003 (2nd version) - '84 to '95 truss-rod adjuster at headstock end one-piece pickguard
4003S (1st version) - '81 to '84 truss-rod adjuster at body end two-piece pickguard
4003S (2nd version) - '84 to current truss-rod adjuster at headstock end one-piece pickguard
["The Bass Book" by Bacon and Moorhouse, as quoted by Matthew Thallmayer,
[email protected], 3/12/1998]