You didn't pay attention for 7-10 years, whose fault was that?

They can say you are a con artist, waiting until the retention period expires before raising your claim.

The issue here is NOT whether you can keep your 500 shares for 7-10 years, the issue is why you didn't raise the claim of the 100 missing shares earlier.  I assume you received quartlerly and annual statements.  

If you put $500 dollars in a bank in 2005, received statements year after year saying only $400 left in the account, then and if now you went to bank and asked for the missing $100, what would the bank say?  Tough luck.  It is the same issue in cash deposit as in stock certificates. 

Good luck.