i believe they are all over the place and different models have different stories. the origin of the bike also may differ from one model to the other. I have a '77 Sprint V made by Dan Motor. it has trapezoid head but its a legit sprint v. with block letterings. the history on the super (VBC) is even more confusing. the super has P/ logo from '65 - '67 and Hexagonal logo from '68 - '79. However, it has a dark blue "Vespa Super" script emblem on the legshield from '66 - '67, a silver "Vespa Super" script from '68 - '72 and a block 'VESPA' from '73 - '79.
we assume that P/ goes with the 'Vespa' script on the legshield, and hexagonal goes with the block 'VESPA' but this certainly not the case because from '68 - '72 they use a hexagonal logo with script badge.
i believe never the opposite, they never use the P/ with block lettering on the legshield.
so to answer your question, i believe sometime around '73 they switch to block lettering.
here's my '69 super (VBC) with script
and my '77 sprint v. (VLB)