Very interesting chart, indeed. Thanks for sharing.
1983-1985 proof pandas are considered as key dates for silver panda series. Based on the charts, the frequency of transaction is more than 10 per year for 1983 and 15 per year for 1984 and 1985. This suggests that they may not be as scarce as some key date gold pandas.
If it is possible, please chart the data based on the different regions, US, Asia and EU, to examine if there is price differences in these three locations. It was my impression, due to the distribution channels of earlier MCC, the price of earlier MCC was lowest in EU and highest in Asia prior to 2011, before the peak of last MCC market circle. Chinese dealers have been actively buying earlier MCC in EU and US during last few years, the price differences may not be there anymore.
Since grading MCC in EU and Asia was not popular prior to 2012 and your chart is based on PR69, there is good chance the regional chart, due to very few data points, may not give any indication of price difference for pre-2012 transactions.