Hi, in case it's not too late to join the party...
What confused me is that the key in the upper left corner of the keyboard did nothing at a prompt, but it works in word processing programs. There are two missing characters (left and right single quotes) and a few ways of typing characters don't work, but the characters can be typed by other means. So there you have it, the USX keyboard.
The reason why the USX keyboard layout you tried works differently at a prompt and word processing programs (which I assume are PM applications) is because text sessions and PM use different implementations of any given keyboard layout, and it turns out both versions of the MCP2 "USX" differ from the one in the image in different ways:
-The MCP2 / ArcaOS 5.0.7 USX layout has combining- and non-combining versions of the acute accent and dieresis reversed wrt what you can find in the image (key to the left of Enter, with 4 symbols on it).
-Additionally, characters (dead keys) do not work in the VIO implementation, though there is no technical reason for that.
-As an extra, the MCP2 USX yields ƒ for Alt+Gr and ª for AltGr+K both in PM and VIO implementations.
As for the 'missing' left and right single quotes, these characters are present in some OS/2 codepages (f.e Win1252/1004, which Windows defaults to), while not in others (f.e. 850, which OS/2 typically defaults to). So if you try and type them in an OS/2 program you get them if typing while using CP1004, but you get nothing if running under CP850. PM vs VIO is not relevant regarding this, as codepages are the same.
[...] (usx) which is not supported under OS/2. Only US International (ux) can be configured.
This is not completely accurate. You can't configure USX as the system keyboard layout from the Keyboard object like you can do with UX, because it is not listed there, but as Daniel mentioned before, you can do it from a command prompt or f.e. a program object that runs "keyb usx", with the limitations I listed above.
I don't know if ArcaOS ships with support drivers that can handle newer keyboard layouts?
Newer keyboard layouts can be added to OS/2 and thus to ArcaOS, although this is not trivial (but not related to 'drivers' either). Sufficient interest might spark the inclusion of tools to handle such new layouts, though.
There are also hints of USY and USZ keyboards in the files that don't seem to actually exist as if someone stopped work suddenly. Or I could be totally wrong...
You are about "USY" and "USZ" keyboards: there is no such thing. This is the full list of keyboard layouts defined in the MCP2 KEYBOARD.DCP file, with some additional data:
Name/def|available CPs (and layers) by keyboard type; 1 = 100+ keys, 0 = 80+
----------------------------------------------------------------------------
AR 238 1: 437, 850, 864(2); 0: 437, 850, 864(2)
470 1: 437, 850, 864(2)
BA 234 1: 850, 852
BE 120 1: 437, 850, 859, 1004; 0: 437, 850
BG 442 1: 850, 855(2), 915(2)
BR 274 1: 437, 850, 1004
275 * 1: 437, 850, 1004
BY 463 1: 850, 866(2), 1125(2), 1131(2)
CA 445 1: 437, 850, 1004
CF 058 1: 850, 863; 0: 850, 863
CZ 243 1: 850, 852
DE 453 1: 437, 850, 1004
DK 159 1: 850, 1004; 0: 850
EE 454 1: 850, 922
FR 120 1: 437, 850, 1004
189 * 1: 437, 850, 859, 1004; 0: 437, 850
GK 220 1: 437, 813(2), 850, 869(2)
319 * 1: 437, 813(2), 850, 869(2)
GR 129 1: 437, 850, 1004; 0: 437, 850
HE 212 1: 437, 850, 862(2); 0: 437, 850, 862(2)
HR 234 1: 850, 852
HU 208 1: 850, 852
IS 197 * 1: 850, 861, 1004; 0: 850, 861
458 1: 850, 1004
IT 141 * 1: 437, 850, 1004; 0: 437, 850
142 1: 437, 850, 1004
LA 171 1: 437, 850, 1004; 0: 437, 850
LT 456 1: 850, 921(2)
LV 455 1: 850, 921(2)
MK 449 1: 850, 855(2), 915(2)
NL 143 1: 437, 850, 1004; 0: 437, 850
NO 155 1: 850, 865, 1004; 0: 850, 865
PL 214 * 1: 850, 852
457 1: 850, 852
PO 163 1: 850, 860, 1004; 0: 850, 860
RO 446 1: 850, 852
RU 441 1: 850, 866(2), 1125(2), 1131(2)
443 * 1: 850, 866(2), 1125(2), 1131(2)
SF 150F 1: 437, 850, 1004; 0: 437, 850
SG 150G 1: 437, 850, 1004; 0: 437, 850
SK 245 1: 850, 852
SL 234 1: 850, 852
SP 172 1: 437, 850, 1004; 0: 437, 850
SQ 452 1: 437, 850
SR 450 1: 850, 855(2), 915(2)
SU 153 1: 437, 850, 859, 1004; 0: 437, 850
SV 153 1: 437, 850, 859, 1004; 0: 437, 850
TH 191 1: 850, 874(2); 0: 850, 874
TR 179 * 1: 850, 857
440 1: 850, 857
UA 465 1: 850, 866(2), 1125(2)
UK 166 * 1: 437, 850, 1004; 0: 437, 850
168 1: 437, 850
US 103 * 1: 437, 850, 852, 857, 862, 864, 866, 1004, 1125, 1131; 0: 437, 850
DV 1: 437, 850, 1004
DVL 1: 437, 850, 1004
DVR 1: 437, 850, 1004
X 1: 437, 850, 859, 1004
UX 103 1: 437, 850, 859, 1004