Hello ZacUK,
thanks for the link.
In the meantime I've found this on Internet, which I've never seen before... maybe it's linked to the modern / stylised letters (which I cannot distinguish), while the differences between the two different mints (Shanghai / Shenyang) are clear enough!
http://www.papertoys.com/modernchinacoins/FEN_VARIETIES.html
Best regards
Globetrotter
Coin varieties in French:
https://monnaiesetvarietes.numista.com