Lebanon Revenue stamp ?

[RICHARD]

My question is why was this revenue stamp applied to the back of this cover?  1948 Lebanon  C 122 and C? payed Air mail to Chicago  from Michel Andros in Beirut  A car part dealer ship. They canceled a precancel as well. I wonder if it was used to help pay the airmail amount. But A revenue would be an ill legally used. I see someone listed it as postal tax. That would be new to me? Perhaps a War Tax type charge.