Check your copy of the Charlton guide. There is an explanation of engraved and lithographed printing in the introduction, and a short writeup at the beginning of the Multicoloured Series section as well that should answer your question. Cheers!
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BWJM, F.O.N.A.
Life Member of CPMS, RCNA, ONA, ANA, IBNS, WCS.
President, IBNS Ontario Chapter.
Treasurer, Waterloo Coin Society.
Show Chair, Cambridge Coin Show.
Fellow of the Ontario Numismatic Association.