yeah they are two pieces. the factory assembly manual (sheet 36.00 covers the "a" pillar assembly and finishing. essentially they would weld, solder and finish a section of the "a" pillar between the cab base and the cab top.
on page 37.00 and 38.00, they cover the assembly of the rear of the cab top to the cab base. it's welded along the seam you discovered.
if you don't have a factory assembly manual it might be worth getting one. they have a lot of good information about how stuff went together, or at least how it was supposed to...
