Since there seems to be a lot of confusion about sUA not operating any 737 variant, I wanted to start this thread and provide sources that sUA does indeed now have 739ERs. A couple of ways to identify an sCO vs sUA aircraft.
1. Tail #s:
On united.com sCO operated flights are in the 3401 - 3478 tail # range. sUA are in the 3801 - 3813 tail # range. All sUA aircraft will be in the 8xx tail # range.
2. IFE: Most sCO 739/739ERs have DirecTV some do not. No sUA 739 has any IFE at all
3. Seats: All sCO 739/739ERs have the original CO seats. Some sUA 739ERs have the CO seats, while from tail #s 811 on up will have the new slimline leather seats.
4. Interior: sCO 739/739ERs either have the traditional 737NG interior (tail #s 401 - 444 or Sky Interior (tail #s 445 -478) All sUA 739ERs have Sky Interior.
A couple of sources that does in fact prove that sUA does indeed have some 739ERs :
http://calafa.org/news/union-update-nov-22-2013
http://unitedafa.org/news/details.aspx?id=8428
http://unitedafa.org/news/details.aspx?id=8254
Hope this clears up the issue of weather or not sUA has 739ERs.