The Ahem font was developed by Todd Fahrner and Myles C. Maxfield to 
help test writers develop predictable tests. The units per em is 1000, 
the ascent is 800, and the descent is 200, thereby making the em 
square exactly square. The glyphs for most characters is simply a box 
which fills this square. The codepoints mapped to this full square 
with a full advance are the following ranges:

U+20-U+26
U+28-U+6F
U+71-U+7E
U+A0-U+C8
U+CA-U+FF
U+131
U+152-U+153
U+178
U+192
U+2C6-U+2C7
U+2C9
U+2D8-U+2DD
U+394
U+3A5
U+3A7
U+3A9
U+3BC
U+3C0
U+2013-U+2014
U+2018-U+201A
U+201C-U+201E
U+2020-U+2022
U+2026
U+2030
U+2039-U+203A
U+2044
U+2122
U+2126
U+2202
U+2206
U+220F
U+2211-U+2212
U+2219-U+221A
U+221E
U+222B
U+2248
U+2260
U+2264-U+2265
U+22F2
U+25CA
U+3007
U+4E00
U+4E03
U+4E09
U+4E5D
U+4E8C
U+4E94
U+516B
U+516D
U+5341
U+56D7
U+56DB
U+571F
U+6728
U+6C34
U+706B
U+91D1
U+F000-U+F002

The codepoints which are mapped to something else are the following:

" " (U+20): No path but full advance
"p" (U+70): Path has 0 ascent but full descent
"É" (U+C9): Path has 0 descent but full ascent
Non-breaking space (U+A0): No path but full advance
Zero-width non-breaking space (U+FEFF): No path and 0 advance
En space (U+2002): No path and half advance
Em space (U+2003): No path but full advance
Three-per-em space (U+2004): No path and one third advance
Four-per-em space (U+2005): No path and one quarter advance
Six-per-em space (U+2006): No path and one sixth advance
Thin space (U+2009): No path and one fifth advance
Hair space (U+200A): No path and one tenth advance
Zero width space (U+200B): No path and no advance
Ideographic space (U+3000): No path but full advance
Zero width non-joiner (U+200C): No path and no advance
Zero width joiner (U+200D): No path and no advance
Greek capital letter Upsilon (U+3A5): Thin vertical stripe and full advance
Greek capital letter Chi (U+3A7): Thin horizontal stripe and full advance
"横" (U+6A2A): Thin horizontal stripe and full advance
"橫" (U+6A6B): Thin horizontal stripe and full advance;
"縦" (U+7E26): Thin vertical stripe and full advance;
"縱" (U+7E31): Thin vertical stripe and full advance;
"纵" (U+7EB5): Thin vertical stripe and full advance.

Source: https://www.w3.org/Style/CSS/Test/Fonts/Ahem/