I posted this in the error forum and it was suggested that I post it here: The first two photos are of a 1978 Yugoslavia 10 Dinara coin with a re-punched date banner. I believe the banner was originally made with a punch intended for the 5 Dinara coin and then re-punched with the correct one. Notice that the underlying punch has a thinner X and 1 and a smaller dot, but that the 4 actually has a wider opening at the top and the 9 is about the same. I do not have overlay capabilities, but I am including a third photo of the date banner of a 1975 5 Dinara and a fourth photo of the date banner of a second 1978 10 Dinara. The characteristics of the 5 Dinara banner seem to fit those of the underlying punch on the first coin. These last three photos were all taken at the same magnification (30x).
Your analysis of the Republic Day banner on your coin seems to fit the physical evidence. The slightly greater separation on the right is consistent with the differing arcs/sizes of the banner on each coin. None of my references for Yugoslav coins (several world coin catalogs including Krause) mention anything about such an error or variety. A very nice find on which the nature of the doubling clearly puts the error on the die, and not as the result of machine doubling. However, if it is a discovery coin, placing a value on it is really not possible without selling it in an auction attracting buyers with a strong interest in Yugoslavian coinage. If it is already known, information about it is probably on a Yugoslavian-language website somewhere, but I have no idea how to search for it.
