I have two of the 2020 reprints, not the original 1963 note.
I had a look at this under reflected infrared, backlight and transmitted infrared. Under backlight and transmitted IR there is BWS repeated on the note and the letters appear dark. Under reflected IR the letters are light.
If you look at the note under white light at an angle, on both sides of the note the BWS is raised. I cannot figure out how they did this, but it does not look like a watermark.
Does anyone have a 1963 note to hand, to check it?