Max, you really must start a proper paperweight collection now.
You can kick it off with extensions to the Perthshire range of doorknobs:
PP16 - spoke pattern - 1969 to 1970
PP17 - spoke pattern - 1969 to 1973
PP18 - closepack - 1969 to 1978 & 1984 to 1997 (some with "P" cane)
PP18A - five cluster pattern & "P" cane - 1995 to 1997
PP73 - concentric with pressed fluted outer and "P" cane - 1984 to 1997
PP76 - mini concentric, fluted outer, "P" cane - 1985 to 1997
Info from the 1997 book
The Complete Guide to Perthshire PaperweightsI don't klnow if they had other doorknob designs between 1997 & 2000.
Yours, by the way, is one of the PP18 range.