[Hugh]

At the Versailles Peace Conference following WW1, it was agreed to transfer a portion of Western Hungary to Austria in August 1921. To prevent the transfer, and with the knowledge and tacit support of the Hungarian government, a 'Free Corps' occupied the Hungarian territory intended for Austria and prevented the Austrian Army from entering the area.

For three months, they exercised military and civil power over the area and proclaimed the independent state of Laitabánság. In the end, Austria and Hungary negotiated a more peaceful resolution.

But, during the almost three months of the occupation, the insurgents issued nine series of stamps -- seven of which were put into circulation (to some extent). Most were produced for speculative purposes but some were used as postage. As the Hungarian MPIK catalogue puts it, "Everyday items are rare, philatelic items are more common."

Here is the third series, issued on October 10, 1921.They're all on piece to preserve what is very likely just a courtesy cancel. What makes this particular issue of interest to me, however,  is that the insurgents printed a surcharge on the backs of the stamps. Fifty percent of the face value of each stamp went to the military HQ of the insurgents. I've added a couple of examples from my collection, not on paper, where the black overprint is easier to see. 

Stamp:
Western Hungary / Insurgency [Banate of Leitha = Lajtabánság]
October 5, 1921 / Series: Overprinted on Hugarian Harvesters definitives
Front Overprint, in black: Lajtabánság-posta [Lajtabánság Post]

Two singles showing ...
Back Overprint, in black: 50% felár. / Fövezérség [50% surcharge / Commander-in-cheif's staff, i.e. High Command]
20f, Brown and Green
MPIK 22 and 24