I have never tried to detect US watermarks from the frontside,let alone on paper.They are difficult enough to spot them from the backside ! Issued Feb.28th,1968.
I never had a closer look to the above stamps. Comparing the backgrounds,one might think they are quiet different.
Scott 219d ,NO GUIDE LINE TO BE FOUND BUT A SCARCE ARROW Position R11/R10. This being one of every 400 stamps printed.