Andrey,
Superior Seed company in Serbia carries Volovsko Srce which translate into Oxheart. I have a picture of it but cannot figure out how to post it here.
Anyway the shape of the fruit is not like the one pictured on the Slovenian packet. That picture resembles what I believe to a Bull's Heart which I was told is Russian. I have grown both and they are not the same tomato!!!
To add to the confusion I also have an Italian tomato called the Heart of the Bull and there are quite a few other varities that use the Oxheart or Bull's Heart designation and quite often interchange them.
Would you or anyone else know what the difference is between a Bull's Heart and an Oxheart?
Alex