"Croatia" Overprints Of Yugoslavia, May 10, 1941

These are not listed in Scott or shown on Steiner pages, so I was unaware of them until I saw them from a trader in Austria and found them listed in Stampworld. I have looked at several internet sources, and, although I can find images, I have not found any history or explanation. I realize I have a narrow view with my limited access to other catalogs or specialized publications, but I would be interested in any explanation as to their issue or use. Thanks,

perf12, thanks, I appreciate your help, are these listed in Michel, Y & T, SG, etc.?

This set is commonly referred to as the "Jubilarka" (Jubilee) issue. From "Manual of the Independent State of Croatia - Nezavisna Država Hrvatska, Issues of 1941-45" by Helmut Rommerskirchen:

10 May 1941, Croatian Armed Forces Special Issue
One month after the establishment of an independent Croatia, a special series of stamps was sold over the postal counters in the capital city, Zagreb. This issue was to commemorate the founding of the Croatian Armed Forces. The issue was sold with a 100% surtax, not shown on the stamp, for the support of the armed forces. Again the 1939 regular issue of Yugoslavia was used with a red or blue overprint consisting of the Coat-of-Arms with the letter "U" above it in the upper left; the date of the founding of the Independent State of Croatia, "10. / IV. / 1941", in the upper right; and the country name, "NEZAVISNA / DRZAVA / HRVATSKA", below. In all there were only 5000 sets overprinted because the regular Yugoslavian issue was running out at the postal headquarters in Zagreb. Printing was done by the State Printing Office in Zagreb. Only 3000 sets were sold. After the expiration date for sale, 15 February 1941, the remainder were distributed as follows: 50 sets were given to members of the government and other prominent persons with the remainder then sold to various philatelic dealers abroad at five times face value. The surtax was used for the education of Croatian officers abroad.

The set was extensively forged.