Dave, you are a font of knowledge, "Usually the code on the back is a production date".....that is the sort of detail that could make an innocent's life a mess.
The web page suggests;
you should run metallic pads if:
-You are heavy
-You ride downhill
-The conditions are wet, muddy, etc.
and run resin pads if:
-you are light
-Conditions are dry
-You like lots of initial grab
-Your riding is less hard on brakes i.e. dirt jump/street, XC, etc.
I'm 82kg and tend not to carry much. Is that "heavy". Probably commuting in winter in Ireland is "wet, muddy", or does muddy mean real mud! My commuting tends to be "light rolling".
Probably mixing the two types is not a good idea unless you really know what you are doing.