I think that this may be Scott 528. The quality of the printing leads me to believe that this is an offset issue. While a higher resolution scan would be helpful, I think this is likely a type V, issued in 1920. Which would be in agreement with the 1921 cancellation date. Hopefully one of the Washington-Franklin experts will chime in and tell both of us what it really is. Added: I really need to read postings more carefully. The offset varieties are not watermarked. So you can ignore my guess! Moreover, type V is Scott 527 rather than 528, so even if I had been right about it being offset type V, I would have had the id wrong.
This is an off-set printed stamp. All off-set printings were on un-watermarked paper. Did you put the stamp in lighter fluid or watermark detection fluid to determine the watermark or are you used some other method to see a watermark?