My precis of Mackay's characterisation from 1900 are:
Type N issued 1900, Mainly London, Registration and Express Fees, For Postage Stamps moved back up, LIST. (one dot), Number (no dot), Postge. (one dot)
Type O issued 1900, LIST (no dot), Number. (one dot), Postge (no dot)
Type P issued 1901, LIST. (one dot), Number (no dot), Postge (no dot)
Type Q issued 1901, LIST (no dot), Number. (one dot), Postge. (one dot)
Type R issued 1901, LIST. (one dot), Number (no dot), Postge. (one dot)
The PPL above, used in December 1900, has a dot after LIST, no dot after Number, and a dot after Postge - which I reckon makes it either Type N or Type R. Type N is "mainly" London while Type R he dates as issued in 1901.
Can anyone help ? Is there another list of types of Parcel Post Label I can consult or an updated version of Mackay's book ?