Actually, early flat plate printings are collected as blocks of six
Parcel Post (Q1-12) are flat plate and the Plate Number Blocks are collected as four, six or 8 depending printing sheet location and inscription.
But I do agree that in the absence of any margin inscription, blocks of six with the number centered is how such are collected. That said, a single number centered on a block of six is only one of the many Scott Specialized listed plate block varieties. Durland is even more detailed listing 45 basic plate number/inscription combination varieties with positional variations not included in that number basic number. Of course, plate numbers on coils are a different game all together with its own long list of possibilities, more if you add in the backside numbering.
Now back to the OP's 1552, that was the very first issue not printed on just stamp paper with connected designs separated by perforation, rouletting, or scissors. Rather the printed stamp paper was separated into individual stamp and text paper wafers adhered to a backing sheet. Now 47 years and 4+ months later the consensus is almost finalized. However the backing sheet was not the only "new fangled" problem with the November 1974 issue.
1968 saw the start of plate numbers exceeding the coverage offered by a block of four. This was the start of the "floating" plate number mess. Walt Disney had multiple plate number but they all fit on a block of four. But the Christmas Toys did not and so began the expansion of the block size for non-floating issues. Only a handful of the large than four blocks were issue by the time 1552 arrived. The next issue faced and addressed was the different designs being more than four on a sheet. Thus then a plate number "block" was considered the selvage with the numbers plus as many extra stamps as needed to get all varieties on the pane. With the 50 State Flag or 50 Bird and Flower (and others) plate blocks of 50 were considered correct. Yet there were and are some who just save the block associated with the plate numbers in the selvage and two adjacent rows or columns. (Here I will give a tip of the hat to the plate number single collectors who do it their way).
In short when Harris issued the pages, no matter what they did, some collectors felt left out or overwhelmed. It was this expansion of the size of plate blocks which, at the time, was considered the main reason for a loss of interest in plate block collecting.
Edit: I started this post over three hours ago but variously interrupted by the honey do bee me
exercises in home-life harmony. Thus any overlap with the prior two posts was unintentional not to mention lifesaving.