The behaviour of the H-2 NMR line-width throughout the whole mesophasic range of the ferroelectric liquid crystal "1-methylheptyl 4'-(4"-n-decyloxybenzoyloxy) biphenyl-4-carboxylate" (10B1M7) is here reported and discussed. Static Fringe Field NMR Diffusometry measurements from the isotropic to the anticlinic phase have been performed leading to a significant result concerning the tumbling diffusional coefficient (D-perpendicular to). These results have been compared to X-Ray measurements and H-2 NMR linewidths in order to explain the peculiar behaviour occurring at the transition. between the ferroelectric (FLC) and the antiferroelectric (AFLC) liquid crystalline phases.