It's Sorta Like This

The rule change started around 1966.  Note that modelers often assume hard and fast dates applied when they really didn't.  Transition periods saw new styles introduced early and old ones lasting after they weren't supposed to.  Old stencils weren't always covered by new ones, as the dates in the originals were just updated to reflect maintenance.

The initial appearance was small black boxes as shown above.

The roughly 1972-1974 stencil was one box with a white border.

From about 1974-1982 two boxes were the norm.  They could be stacked like this, or side by side.  They could also be separated around ribs, flat car stake pockets, or whatever.

A three box design like this became standard in about 1982, and remains to this day.  There was an interim type with four boxes, like this one but with three adjacent on top instead of two.  That one didn't last long, but showed up in around 1981.

See  http://www.hosam.com/mod/rsdet.html 

Nice