In my experience, if you look at multiple ORERs over the years, the outside dimensions are often inconsistent, especially the various heights.  I'm not really sure what changed -- conventions, hardware on the car, other?  I also have seen examples of that H at the EXW being quite low (~5 ft), but am not sure I have seen it in an actual car stencil.

Whenever possible, I follow what's in a period-appropriate photo.  Absent that, I may go with what's in an ORER if it seems to make sense.  Absent that, I fake it.  If I don't know what the correct dimension is, what are the odds that somebody else will (and notice)?
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Dave Parker
Swall Meadows, CA