PP1 was large size (3" = 7.5 cm) and pre-1978 weights had a fire polished base and no P cane; number of twists, colours and canes varied.
PP6 until 1971 was only medium. 1972 to 1977 it was large, same design as PP1 ... but:
- central P cane, and
- base hollow ground.
PP6 was discontinued after 1977, when PP1 became signed and with hollow ground base.
Conclusion: the weight in question is a pre-1978 PP1.