This sounds confusing to me: have things changed these last years? The 15.... key is my public address right? That should not change when I make a transaction is it? Maybe I was never aware how a transaction really took place.
How can I check this in Electrum?
And don't forget to get the altcoins you also have from blockchain splittings that passed recently, BCH (bitcoincash) and BTG (bitcoingold), both at a 1:1 ratio for the BTC you have, and BCD (bitcoindiamond) at a 10:1 ratio. If you use an android phone, consider Coinomi as wallet, covers most cryptocoins but BCD not yet.