The global hycom grid is rectilinear from j=1 to j=2172, and you can check this by confirming that the minval and maxval of plat(1:4500,j) are identical there. The grid is curvilinear (bi-polar patch) from j=2173 to j=3298. Note that the j=3298 row is a permuted copy of j=3297 (because of the way the tripole grid is implemented in HYCOM), so you can discard j=3298.