John Lennon autographs - what's the secret?

I guess like many people, my visits to this site increase when I have just bought something or am considering buying something. This is one of those times.

I have just bought a set of Beatles autographs, or rather a set of three and a separate Lennon index card that was originally bought to "make up the set".

I am pretty confident about the three that were obtained "in person" but had, and still have, significant doubts about John's. I'm 100% sure the person who put the set together thought the Lennon was genuine (he is no longer with us) but as it was obtained from an unnamed dealer before the age of the internet he may not have seen many - or any - examples before buying it. If it is "right" it is certainly from the mid- to late 70s.

Over the last 10-20 years I have looked at many hundreds of Lennon autographs in auction catalogues and on the internet but I am still not really sure about his late 60s and 70s signatures. They seem extraordinarily different to the early/mid-60s examples and even differ greatly amongst themselves.

I have four questions, two "serious", one "out of interest" and the one "just for the sake of it":

1.  What do other members think are the characteristics that never changed in John's signature - those that would be tell-tale signs that an unusual example (like mine) may be "right". If there are already good articles on this, particularly illustrated ones on the internet, I'd be grateful for details of them.

2. Was John a prolific signer in the mid- to late 70s and is there any history of him signing index cards "in person" or in response to written requests? I did see an index card example from late 1980, with some similarities to mine, that sold back in 2002.

3.  Why do members think that John's signature changed so much over the years? The signatures of the other three look pretty similar all the way through, if you discount the very early efforts and the rushed examples.  

4. How many times do members think that John (or the other Beatles) signed their names? It hardly bears thinking about.

I may pluck up courage and upload an image of the Lennon but I'd rather do a bit of research myself to start with.