Seventy years of marriage is the platinum wedding. It qualifies for a message from the King, and from the 70th on, every anniversary does.
The bit almost every guide truncates
Search this anniversary and you will be told that the King sends a message for the 60th, the 65th and the 70th. That list is correct and it is incomplete. The wording is "diamond wedding (60th), 65th, 70th wedding anniversaries and every year thereafter".
So a seventy-first qualifies. So does a seventy-second, a seventy-fifth, an eightieth. After seventy the milestone stops being occasional and becomes annual, which is the opposite of what a list of three numbers implies - and for a family with a couple in their nineties, it is the difference between applying once and applying every year.
The application windows, the marriage-certificate requirement and the fact that the Palace makes no charge are set out on the diamond wedding page rather than repeated here.
Platinum is a hallmarked metal, with the lowest exemption of the four
Platinum is one of only four metals UK hallmarking covers, alongside gold, silver and palladium. It also carries the lowest exemption threshold of the four: an article is exempt from hallmarking if it contains less than 0.5 g of platinum, against 1 g for gold, 1 g for palladium and 7.78 g for silver. (Source: Business Companion.)
That matters more than it sounds. Platinum is dense and expensive, so a great many platinum pieces are small - and a small piece is exactly the one most likely to fall under the threshold, where a purity stamp is the maker's own claim rather than an Assay Office test. The metal with the highest price per gram is the one where a light article is least likely to have been independently checked.
The platinum standards recognised under the international Hallmarking Convention, as published by the Goldsmiths' Company Assay Office, are 850, 900, 950 and 999. What each of the five component marks proves is set out on the silver wedding page; it reads the same on platinum.
What seventy years gets given
Buying platinum at this anniversary is an expensive way to honour a word. Almost nobody does it, and nobody expects it.
What a seventieth tends to want is the opposite of new: the photographs found, copied and made permanent while the people in them can still tell you who everyone is. At seventy years the wedding picture is often a single fragile print, and it's the only one.
Working backwards: the 65th has no material on the British list at all, and the diamond wedding at sixty is where the messages start. Every year is on the UK list.
Anniversary material from Debrett's; hallmarking exemptions from Business Companion; platinum finenesses from the Assay Office, as Convention standards; Royal message rule from royal.uk.