While I am here, I will answer your question about Strathearn numbering and dates.
1. Yes, they had a numbering system for design types. Yours is either a P7 or a P8 depending on the size. P7 was 2 3/4 inch to 3 inch, P8 smaller.
2. The date of your weight could be anywhere from 1965 to 1980 as it is was a regular production design.
The best information on the designs and dating is in Richard More's site at:http://strathearn.smugmug.com/Strathearn%20Paperweights
(which is the page for the design listings with numbering).