Very interesting...I have been in a similar situation, which is odd because you would think much of the feed for the flight comes from UA pax at IAD (in the OP's case).
On a separate note, if they did ticket on TK ticket stock...would UA know if you earned US miles on the ticket if it were not a -016 ticket? Not that I am advocating doing this
, but just wondering since this is a transition period for US...could someone traveling during this time period potentially double dip? or does UA have a way to find out if it is issued on TK's ticket stock?