Reconstruction of square US standard road sign lettering (1927) by Eric Fischer on Flickr:
Overlaying the 1952 Standard Alphabets for Highway Signs with as many characters from the earlier square-letter standard as I could find in sample signs from pre-1942 editions of the MUTCD, mostly from the 1927 Rural Sign Manual. The real standards for these characters, never published in book form, probably still exist in a dusty file cabinet in some public works department office.
I was not able to find any square letters from Series A, only a few digits. The 1935 MUTCD specifies “Soft Shoulders” to be in Series A, but the image of the sign appears to be in Series B. I didn’t find Q or 9 in any series.
In general the metrics of the square characters closely match those of their rounded successors, much as Clearview has tried to do. The exception is W, which was approximately a series narrower in the square letters.
The Series B period (from 7 A.M.) seems to be round and the Series C period (from St. Louis) square, for no obvious reason.