Rather than revenue used as postage or the reverse, this is the case of one revenue type being used improperly as another.
RB1a used illegally on document along with two R135 (including one with margin imprint capture) and an R24c, paying the correct 10-cent tax on a $175 promissory note.
The piece cut out at bottom right seems to correspond to stamps that would have paid the tax on the transaction on the other side of the document.
Not bad for $2 shipped.

