Check out http://www.epsg-registry.org/report.htm?type=selection&entity=urn:x-ogc:def:crs:EPSG:31275&entity=urn:x-ogc:def:crs:EPSG:31276&entity=urn:x-ogc:def:crs:EPSG:31277&entity=urn:x-ogc:def:crs:EPSG:31279&reportDetail=long&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=Balkan
If that doesn't work for you, do a web search for Croatia, easting, datum, transverse mercator, and such.
It appears that the Croatians use what the Canadians call 3TM, which is transverse mercator based on 3 degree zones instead of the usual 6. It further appears that Croatia uses much larger eastings (5500000-8500000) than are used in the United States of America (500000). That being said, I was still unable to get (6551335, 5045931) to equal (18 39' 07.7'',45 33 ' 22.0"), although I came a great deal closer than with conventional UTM. I tried DATUM HER instead of the usual NAD27 and NAD83 as well. The Y coordinate seems about right but the X is still way off.