It is well known that some Z80A processors with non functioning M1 signals were assembled into many ZX Spectrums. As this signal is not used in the main board and not everyone bought interfaces that used it, this fault may pass unnoticed during the whole service life of the computer.
Now many of these M1-faulty Spectrums have started to show their condition, as retro users want to use DivIDE or DivMMC interfaces with them. These interfaces do need a working M1 signal.
As the Z80A was second sourced by several brands (Mostek, Toshiba, Sharp, SGS and NEC, among others), I'd like to know which of these brands could be considered more reliable in terms of having a working M1 signal (is it there any statistics or anything?)