Definitely some odd stuff here. Most of this bass looks much later than the headstock stamp implies. The pickups, with the molded in Alembic logo are late 80s vintage (probably 86-87, I think). The selector knob location is like an even later bass (90's), since it was usually on the treble bout. Also, on older basses, I believe the battery cover plate is usually round rather than square. Others have already pointed out that the bookmatch-to-center wasn't happening at the Alembic factory until the late 90's. So, I think you're looking at some very extensive reconstruction.
The peghead break looks pretty serious. If it feels stable, then I guess you'll be OK with it. Normally if this happened, I think the factory would have mimimally stripped off the face veneers of the peghead and replaced them.
David Fung