In the 1st Edition his WW1 book, Proud illustrates an APO P.P.1. type C (type D3 in 2nd Ed) he states that this type C was only used later on the Western Front 1918-19
Attached is a obviously philatelic cover showing the datestamp was used earlier in Italy
In his 2nd Ed Proud changed his reference numbers i.e. types A B C became D1 D2 D3, which means I would have to rewrite 95% of my collection, now I only use the new system on previously unlisted datestamps
Frank Schofield