Hello Elis,
To unlock the security feature of the HC908 device, and access the code within a programmed device using monitor mode, you will need to ascertain the values of the eight bytes from address $FFF6 to $FFFD. These are interrupt vectors, and will correspond to the address of four different interrupt service routines within the code.
These vectors are -
$FFF6 TIM Ch0 vector
$FFF8 IRQ vector
$FFFA USB vector
$FFFC SWI vector
How were the devices programmed in the first instance? If a .S19 file is still in existence, this may be used to program new devices, without the need of the other source files.
Regards,
Mac