Slijk (literal translation: mud, muck ;-) ) is a rare family name in the Netherlands. In 2007, there were only 35 people bearing that surname living in our country. As you can see on the map, the name is concentrated in and around Rotterdam http://www.meertens.knaw.nl/nfb/detail_naam.php?gba_lcnaam=s...
In 1947, the situation was even clearer (see http://www.meertens.knaw.nl/nfb/detail_naam.php?nfd_naam=Sli...
), it definitely looks like this is a family from Rotterdam.
The name "Elleus" doesn't ring a bell ... Could is be a very mangled up version of "Jacobus" maybe?
If you look for the surname "Sli*k* at the archive of Rotterdam (http://rotterdam.digitalestamboom.nl/search.aspx
), then there are numerous hits, but not exactly the ones you're looking for.
Good luck, cheers,