score:1

Accepted answer
var results = from p in orders
              from pre in prefixlist
              where p.ordcust.startswith(pre)) 
              select p;

score:1

        var items = from o in orders
                    where (
                        from pf in xrfprefixes
                        where o.ordcust.startswith(pf.companyprefix)
                        select pf).any()
                    select o;

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