A. Letter height
B. Space between letter and period
C. Between lines
D. Letter spacing
E. Space to bottom of hood edge
F. Overall length
The dimensions and letter types are listed in both of the common directives. AR 850-5 and TB 746-93-1.
AR 850-5 refers to block type letters and TB 746-93-1 refers to Gothic style letters. These are one in the same and are now usually referred to as sans-serif letters.
Generally AR 850-5 covers 1941 thru 1954 and the TB picks up at 1955.
The manuals always offer options based on the available surface. They are rarely specific on measurements form edges of panels and etc.
M38's were stenciled and depending on what day of the week, or which employee or which set of stencils were used the exact location vertically or horizontally will vary.
These Willys factory lot images are your best location guide for the M38:

In this one use your dividers to compare the height of the slave receptacle bracket with the edge distance of the markings from the edges of the hood.
Download your own copies of the two pubs so you can read the fine details in them:
TB 746-93-1 can be downloaded free here:
http://www.armyradio.com/arsc/customer/ ... lletin.htm
http://vietnam.net.pl/TB746_en.htm
AR 850-5 can be downloaded free here:
http://jeepdraw.com/images/AR-850-5.pdf
References to Block style and Gothic styles are easy to google:
http://en.wikipedia.org/wiki/Block_letters
http://en.wikipedia.org/wiki/Sans-serif

Sans-serif font (without-serifs font)

Serif font

Serif font with serif's shown in red
Sans-Serif is Gothic style letters. Block and Gothic really were used by the Army to describe the same letter type which is block or printed type instead of cursive or written script type and without serifs.
You will find the most definitive sizing info on the Gothic.Block/Sans-serif letters in the AR 850-5.
A final note: You will see that centered on the available, unobstructed space on each side of the hood
is not centered between the front and rear edges of the hood. It means centered in that unobstructed space.