1921 Promissory Note With Revenue Stamps

I came across a small stack of these promissory notes with revenue stamps attached. Here's an example of one.





I know the stamps themselves depending on the denomination are R228 to R239.

I'm a little lost in understanding the rates.

I have others that are a a few years later 1926 to about 1933 that don't have any revenue stamps. The paper doesn't show evidence that any were once attached.

What would the ones with stamps attached be valued at? I would assume the non stamped ones are worth less?

I would like to preserve these from potential damage what kind of holder would do this?

Thanks for the assistance. -MV

Promissory note taxed at the rate of 2 cents per $100. Here is the rate chart from the 1920 US Official Postal Guide:

Thanks John that answered the rate question perfectly.

-MV