A deuterium two-dimensional exchange NMR technique is used to study the chiral smectic C (SmC*) and chiral subphases (SMC*(Fi1,) SMC*(Fi2)) of a smectogen (s)-1-methylheptyl 4'-(4-n-decyloxy-benzoyloxy)biphenyl-4-carboxylate (10B 1M7). The aim is to demonstrate how this technique can be combined with sample rotation in the magnetic field to obtain the fast translational self-diffusion constant along the helical pitch in the SmC* phase and to shed light on the structure of helicoidal superlattice in the three-layer SmC*(Fi1) and four-layer SmC*(Fi2) phases. The cross-peak intensities in the 2D exchange spectrum are sensitive to the disposition of molecules in the three- and four-layer base unit. The results support the "asymmetric Clock model" as an appropriate description of the ferrielectric phases in this compound.