I was going to write a post on the March 22 cover celebrating the "50th Year of British Machin Definitive Stamps" as shown on the GBStamp blog, but Roy beat me to it. However, neither the original post nor Roy's post said exactly what this postmark and cover celebrates.
The first three Machins were issued on June 5, 1967. We're clearly not celebrating the 50th anniversary of the first Machins, since that will occur on June 5, 2017.
We're not celebrating 50 years of the use of Machins, if by that we mean 50 periods of 365/366 days each. The first year of use started on June 5, 1967 and ended on June 4, 1968. The 50th year of use starts on June 5, 2016 and ends on June 4, 2017. We've not come to either of those dates.
The only way I can understand how the "50th Year" can be celebrated this early in 2016 is to understand it as the 50th calendar year in which Machins are used. That is, the first calendar year was 1967, the second calendar year was 1968, and so on until we reach the 50th calendar year of 2016. In that case, though, shouldn't we have celebrated it on the first day of the year, January 1, 2016?
So, can someone tell me exactly what March 22, 2016 has to do with 50 years of Machins?