Revenue as Postage: 2 Cent Documentary with Postal Cancel

R164 2 cent documentary used for postage

Clearly used for postage, this 2 cent documentary stamp was canceled with a common postage stamp canceling device of the period.  This use was illegal as I understand.  This stamp has been removed from its cover unfortunately.  Cover markings would tell us whether or not the letter actually went through or was returned or held for proper postage.  Is there any legal reason why a documentary stamp could have been used on mail during this tax period?  I know there were postal uses for documentary stamps during the World War I tax period, but I am unfamiliar with any uses for the 1898s.