Well, Delphi is different than Delfi. I guess you mean the latter.
I believe that starting with version 5.1, the author went to a commercial model, and the free 5.1 version can only be set to either full strength or 1000 elo.
Ver 4.6 :
http://web.archive.org/web/20060506112927/http://www.msbsoftware.it/delfi/
Ver 5.0 :
http://web.archive.org/web/20070208171602/http://www.msbsoftware.it/delfi/
And for what it's worth, Ver 5.1 (free) :
http://web.archive.org/web/20070222081342/http://www.msbsoftware.it/delfi/
I'm searching for old versions of Delphi chess engine, specifically 4.6 and 5.1.
Anyone have them ?