  Vertical van der Waals heterostructures of semiconducting transition metal
dichalcogenides realize moir\'e systems with rich correlated electron phases
and moir\'e exciton phenomena. For material combinations with small lattice
mismatch and twist angles as in MoSe$_2$-WSe$_2$, however, lattice
reconstruction eliminates the canonical moir\'e pattern and instead gives rise
to arrays of periodically reconstructed nanoscale domains and mesoscopically
extended areas of one atomic registry. Here, we elucidate the role of atomic
reconstruction in MoSe$_2$-WSe$_2$ heterostructures synthesized by chemical
vapor deposition. With complementary imaging down to the atomic scale,
simulations, and optical spectroscopy methods we identify the coexistence of
moir\'e-type cores and extended moir\'e-free regions in heterostacks with
parallel and antiparallel alignment. Our work highlights the potential of
chemical vapor deposition for applications requiring laterally extended
heterosystems of one atomic registry or exciton-confining heterostack arrays.
