Hungary Charity Overprint From WWI

I have two cards from Hungary that appear to be some sort of event commemoration.

I was able to glean this information from http://www.stamp-collecting-world.c..._1913ch.html:
""The two postage stamps of Hungary shown above were overprinted and surcharged on September 15, 1917 in connection with the War Exhibition of Archduke Josef, with the rather large surcharge benefiting the war effort.
The stamp design depicts peasants harvesting. Each stamp features the Hungarian overprint "József fõherceg vezérezredes hadí kiállitása 1 korona" meaning "Archduke Josef military leader 1 Korona", thus each stamp had a one Korona surcharge.""

That site indicates that the surcharges were made on 15 September 1917. However, the two cards are dated 20 September 1917 and 21 August 1918. Perhaps these were exhibition dates? Anyone have any further information?

The site your refer is wrong; 15 September 1917 is not the date the stamps overprint was made, it's the first day of release of those 2 stamps with this particular overprint.


First series was sell to public from Sept 15 to Oct 15 1917 and later was sold betwen May 1 to Oct 30 1918

Thanks area66. The covers are clearly philatelic. Any idea on the significance of the two dates?