Made the following update to the Alto in Treatises page, giving a summary of specific keys that writers have historically assigned to the alto trombone. For more on alto trombone keys, see also the Extant Altos page, which lists alto trombones manufactured before 1800, along with their respective keys.
OVERALL KEY TOTALS:
40 D/E-flat
4 F
2 B-flat
1 E
GERMANY: 14 D/E-flat, 1 B-flat
Praetorius (1619)—D
Speer (1697)—D
Niedt (1721)—D
Majer (1729)—D
Christoph and Stössel (1736)—D
Eisel (1738)—D
Koch (1802)—D
Fröhlich (c. 1811)—B-flat/E-flat (contradictory)
Riemann (1882)—E-flat
Saro (1883)—E-flat
Jadassohn (1889)—E-flat
Schroeder (1889)—E-flat
Hofmann (1893)—E-flat
Mayerhoff (1913)—E-flat
UNITED KINGDOM: 10 E-flat, 3 F, 1 E
Bertini (1830)—E-flat
Mandel (1859)—E-flat, E, F
Prout (1877)—E-flat
Stone/Grove (1879-90)—E-flat, F
Hunter (1879-88)—E-flat
Encyclopaedia Britannica (1888)—E-flat, F
Corder (1896)—E-flat
Vincent (1897)—E-flat
Fitzgerald (1902)—E-flat
Forsyth (1913)—E-flat
FRANCE: 5 E-flat
Braun (c. 1795)—E-flat
Kastner (1839)—E-flat
Berlioz (1844)—E-flat
Gevaert (1863)—E-flat
Lavignac (1895)—E-flat
AUSTRIA: 1 E-flat, 1 B-flat
Seyfried/Albrechtsberger (1826)—E-flat
Nemetz (1827)—B-flat
BELGIUM: 3 E-flat
Mahillon (1897)—E-flat
Ergo (1908)—E-flat
Gilson (1921)—E-flat
ITALY: 1 E-flat
Galli (1898)—E-flat
US/CANADA: 6 E-flat, 1 F
Coon (1883)—E-flat
Claus (1884)—E-flat
Olker (1889)—E-flat
Henderson (1899)—E-flat
L. Elson (1900)—E-flat
A. Elson (1922)—F
White (1924)—E-flat