plum tree - copper (or iron?) nail?

I was on the phone to mum last night and I mentioned that I couldn't decide on what variety of plum I wanted for the allotment and if she could remember the variety my granddad had as I couldn't remember
(he planted the plum when me and my cousin were born to celebrate (there are only 3 days age difference between us) and I remember they were delicious, the tree was only 7-8ft (ideal) and so laden with fruit he had to prop the branches up to stop them snapping!)

Well, the variety was 'Victoria' "because it's the best" and she also said "you must remember to carefully hammer a copper nail into the trunk, Granddad used to swear it was why he has so much fruit!"  My Dad had apparently found the nail for him as they are hard to come by...

I was quite surprised so have done a bit of researching and have only found reference to copper to kill a tree.... BUT, I have stumbled across a  few mentions of hammering an Iron nail into fruit trees to increase fruit production, or even putting them into the soil around and under the tree when you plant it -
(It has been common for my Mum to get herself mixed up with things so I wouldn't be surprised if she meant iron, not copper) 

Which leads me to the question - Has anyone tried or heard of doing this? And, what other  wise words of advice am I missing out on??
   

copper nails are sometimes recommended for killing trees 

'A woman ,a dog and a walnut tree,the more you beat them the better they be'

A very old saying, not sure I hold with all of it though