Note that this index is still an index on the SerNr field. So you should start with that as -1.
Note also the following code in master:
function LongInt ORVcRecordInIndex(record ORVc ORr,string indexname)
if (ORr.OSFlag==0) then begin
if (indexname=="ActArtCodeOSFlag") then begin res = 0; end;
end;
So only records with OSFlag <> 0 will be listed in that particular index.