I have to admit I'm a bit of a pedant when it comes to history - it comes from working in Film restoration and in the media. There's a lot of revisionist thinking out there (re-writing of history).
So what was the 'Puffin Club' (not Puffin Post)? Well here's a great little explanation by Kaye Webb herself, writing in the Puffin Diary of 1979:
The new club is called 'Puffin Post' - not the 'Puffin Club'. The club is now the schools entity. I suppose this makes it easier to differentiate between the two. I can't help feeling this may confuse a few of us older members, but I have to remind you - 'It's not for us! It's for our kids!'