I lost 6 years and didn't get a letter.
If we weren't worth the cost of a stamp, there could have been public service announcements between TV shows. 'Wear a hat, and keep your head', or 'Learn to swim, young man, learn to swim', anyone? There were PSAs for everything from children not playing on railway lines to staying in lane on motorways. Why not let people know that way? Could it be that it would be poor PR for any government in power at the time?
I knew about the changes - I'm not 100% sure how, but I have always been in a union, and suspect that they would have let members know. I did read financial pages, but I know not everyone did.
I also know, however, that many women had no idea - sometimes they found out after resigning from their jobs expecting to get a pension. These days it is easy to check things online, but that was not always the case, and it is very possible that those who came late to the Internet are more likely to be those who didn't read papers or watch news-based TV.
I always ask this, but never get an answer. Why do those who doubt women who tell them they were unaware think they are doing so? What will they stand to gain? If there is compensation it will go to everyone, whether they know or not - it would be impossible to evaluate individual cases.