I have been bitten by a NN bug, and have been accumulating NN a little by little

I remember reading about the horse-shoe shape on the end of the barrel, inside the indentation but cannot seem to find the thread

Is it correct that the absence of horse-shoe shape indicates older NN pens? Also I've noticed that pens without the horse-shoe shape has just "Sheaffer" on the clip. So, are pens with engraving "Sheaffer U.S.A." on the clip newer pens?
Thanks in advance!!