I found in the naturalization files of both my great-grandfather and of my grandfather, that they published, as was stipulated by law, in a local newspaper an announcement in which they publish their intention to be naturalized as Belgians.
Part of naturalization file 21875N from my great-grandfather in which it is written when the announcement appeared in the "Le Matin"
Before they naturalized they were stateless citizens because when my great-grandfather Gerschon Lehrer was born in Ustrzyki-Dolne, it was Austrian. After the First World War it became Poland. My great grandfather lived at that time in Dresden (Germany) and Poland took away the nationalities from all citizens that were not in the country for a long time, hence my great-grandfather and grandfather automatically became stateless (source: naturalization file 21875N).
(update April 11th, 2011 -> check my new post “Citzenship status of Galician Jewish refugees after World War I“)
I was trying to find the newspaper. In the naturalization papers I read that my great-grandfather published the announcement in the “Le Matin” (on the 15th of June 1952) and my grandfather published it in the “Het Handelsblad” (no specific date was given).
I’ve been told after contacting a few organisations, that the best place to look is the “Erfgoedbibliotheek Hendrik Conscience” which is in the centre of the City of Antwerp near its universities (their website is at http://www.consciencebibliotheek.be).
That library collects everything which was ever written on a Flemish subject. They also try to collect all Flemish periodicals and newspapers. Some of the newspapers are bundled in books and some are microfilmed.
When you want to check out their catalog, you can do so before you head to the library at this URL: http://anet.ua.ac.be/desktop/ehc.
I did succeed in finding the artcile in the “Le Matin”, I however did not find yet the article of my grandfather which did appear in the “Het Handelsblad” because I do not know the exact date of when it appeared in the newspaper. To find it, I’ll need to go through some newspapers (which requires time and patience).
The announcemet in the "Le Matin" of June 14th and 15th on page 15
It is not critical for my research to find these announcements as everything I want to know appears already in the naturalization files, but it is still nice to have it.