Hi @hasu. I managed to change keymap_common.h and make the firmware .hex work with my M3501 British Keyboard. As you linked to wikipedia earlier, Apple British keyboards are not ISO standard. I made a simple change to swap K32 and K0A keys reversing your commit made in 2016. Now the keys work everytime even when I disconnect the USB cable. Perhaps we need a ISO/UK section. I had a look at the keyboard-autodetection code and there's no accounting for UK keyboards.
/* M3501 Apple Extended Keyboard II UK ISO
* ,---. ,---------------. ,---------------. ,---------------. ,-----------. ,---.
* |Esc| |F1 |F2 |F3 |F4 | |F5 |F6 |F7 |F8 | |F9 |F10|F11|F12| |PrS|ScL|Pau| |Pwr|
* `---' `---------------' `---------------' `---------------' `-----------' `---'
* ,-----------------------------------------------------------. ,-----------. ,---------------.
* | §| 1| 2| 3| 4| 5| 6| 7| 8| 9| 0| -| =|Backspa| |Ins|Hom|PgU| |NmL| =| /| *|
* |-----------------------------------------------------------| |-----------| |---------------|
* |Tab | Q| W| E| R| T| Y| U| I| O| P| [| ]|Retur| |Del|End|PgD| | 7| 8| 9| -|
* |------------------------------------------------------` | `-----------' |---------------|
* |CapsLo| A| S| D| F| G| H| J| K| L| ;| '| \| | | 4| 5| 6| +|
* |-----------------------------------------------------------| ,---. |---------------|
* |Shif| `| Z| X| C| V| B| N| M| ,| ,| /|Shift | |Up | | 1| 2| 3| |
* |-----------------------------------------------------------| ,-----------. |-----------|Ent|
* |Ctrl |Opt |Cmd | Space | |Opt |Ctrl | |Lef|Dow|Rig| | 0| .| |
* `-----------------------------------------------------------' `-----------' `---------------'
*/
#define KEYMAP_EXT_ISO( \
K35, K7A,K78,K63,K76, K60,K61,K62,K64, K65,K6D,K67,K6F, K69,K6B,K71, K7F, \
K32,K12,K13,K14,K15,K17,K16,K1A,K1C,K19,K1D,K1B,K18,K33, K72,K73,K74, K47,K51,K4B,K43, \
K30,K0C,K0D,K0E,K0F,K11,K10,K20,K22,K1F,K23,K21,K1E,K24, K75,K77,K79, K59,K5B,K5C,K4E, \
K39,K00,K01,K02,K03,K05,K04,K26,K28,K25,K29,K27,K2A, K56,K57,K58,K45, \
K38,K0A,K06,K07,K08,K09,K0B,K2D,K2E,K2B,K2F,K2C, K7B, K3E, K53,K54,K55, \
K36,K3A,K37, K31, K7C,K7D, K3B,K3D,K3C, K52, K41,K4C \
) { \
{ KC_##K00, KC_##K01, KC_##K02, KC_##K03, KC_##K04, KC_##K05, KC_##K06, KC_##K07 }, \
{ KC_##K08, KC_##K09, KC_##K0A, KC_##K0B, KC_##K0C, KC_##K0D, KC_##K0E, KC_##K0F }, \
{ KC_##K10, KC_##K11, KC_##K12, KC_##K13, KC_##K14, KC_##K15, KC_##K16, KC_##K17 }, \
{ KC_##K18, KC_##K19, KC_##K1A, KC_##K1B, KC_##K1C, KC_##K1D, KC_##K1E, KC_##K1F }, \
{ KC_##K20, KC_##K21, KC_##K22, KC_##K23, KC_##K24, KC_##K25, KC_##K26, KC_##K27 }, \
{ KC_##K28, KC_##K29, KC_##K2A, KC_##K2B, KC_##K2C, KC_##K2D, KC_##K2E, KC_##K2F }, \
{ KC_##K30, KC_##K31, KC_##K32, KC_##K33, KC_PENT, KC_##K35, KC_##K36, KC_##K37 }, \
{ KC_##K38, KC_##K39, KC_##K3A, KC_##K3B, KC_##K3C, KC_##K3D, KC_##K3E, KC_NO }, \
{ KC_F17, KC_##K41, KC_NO, KC_##K43, KC_F18, KC_##K45, KC_NO, KC_##K47 }, \
{ KC_NO, KC_NO, KC_NO, KC_##K4B, KC_##K4C, KC_NO, KC_##K4E, KC_F18 }, \
{ KC_F19, KC_##K51, KC_##K52, KC_##K53, KC_##K54, KC_##K55, KC_##K56, KC_##K57 }, \
{ KC_##K58, KC_##K59, KC_F20, KC_##K5B, KC_##K5C, KC_INT3, KC_INT1, KC_PCMM }, \
{ KC_##K60, KC_##K61, KC_##K62, KC_##K63, KC_##K64, KC_##K65, KC_LANG2, KC_##K67 }, \
{ KC_LANG1, KC_##K69, KC_F16, KC_##K6B, KC_NO, KC_##K6D, KC_APP, KC_##K6F }, \
{ KC_NO, KC_##K71, KC_##K72, KC_##K73, KC_##K74, KC_##K75, KC_##K76, KC_##K77 }, \
{ KC_##K78, KC_##K79, KC_##K7A, KC_##K7B, KC_##K7C, KC_##K7D, KC_NO, KC_##K7F } \
}
