First of all, hi everybody!
Here comes a little oddity:
As you know, Germano-Balto-Slavic group uses some kind of *sirabr- stem instead of general IE *arg- . I don't think it's really a loan from "Pelasgian" related to that "Sibro" river. I recently read about Basque "zilar" for "silver" and protobasque "zirar" and probaly an unattested *zirabr-. I don't know how Basque "z" must be represented, but I can tell you that it is kind of an "s" executed with the tongue tip just opposite to the frontal teeth. It is like /th/ but without separing your teeth (god! it's so difficult!). What I mean is that it's rather like an "s" than an English "z".
I also found a web-site about euskara which mentions this and other cases in which IE and euskera seem to have had ancient contacts (besides other general aspects concerning this jewel-language):
http://free.freespeech.org/ehj/html/frbasque.html
There's another odd fact about these stems: euskeric word for "light" (adjective as well as noun) is "argi-a"! (Here most linguists agree that it is an IE loan... but anyway it looks interesting, don't you think?)