Between Mid '75 and '77 there were four distinct changes:
Pickups
Channel Bridge
Solid Brass Tailpiece
Dual Truss Rods
I believe they occurred in that order. Perhaps due to a surplus of parts, I've yet to see a four-screw pup guitar prior to '76.
I'm not sure exactly when the channel bridge was phased out, but every '77 I've seen has the new style, same w/ the truss rods.
75-241 was sort of a mystery because it had four-screw pups, solid tailpiece, and post '77 bridge. But once I was under the hood, it was apparent that the bass made it's way back to the mothership at some point~ the pups being original, everything else upgraded.