Nickname Seka is really common, both in Serbia and Croatia. I woud say that our Seka is Serbian, with a summer house in Croatia, which used to be usual for wealthy family, Croatia or Montenegro.
Sigrid is not a usual name, we, from ex Yugoslavia, would say, a foreign name, not "our name", norevgian, swedish...most probably.
It woud not be hard to track, but we need her last name. In that case I could track her in a day. I would say that she was wealthy because in 1976, in a comunisam era in Yugoslavia, she spoke English, had a summer house, and, if she was a student or postgraduate or similar, most probably woud be medicine, technical or similar, no social science for sure...
I really hope that we will have some response...I mean, she is near 60, small number of persons fits her decription...
I am exploring some new ideas also, will share with Annasmom and you...
I do not how but you can bet that we will find her!!!
~~~
Miss Marple, again thank you for your enthusiasm, you've got me pumped!
On the letter, I will have to go and have another look, the post mark Belgrade, and Skradin the smaller town, are these near Montenegro? Seka's family would be residents of this town perhaps.