Took a guess based on OP's program listing of LO SEN that the route is WAWLAS. There is effectively a OS
ELRCEU3W fare filed for that route.
Looks like it books into M on UA.
Code:
057 WAWLAS 20MAY18 OS USD 1208.00 ELRCEU3W STAY- V/12MBK-E
FARE CLS EXPLANATION BOOK CODES
-------- ---------------------- ----------
ELRCEU3W PREMIUM ECONOMY ADVANCE PURCHASE FARES E
ELRCEU3W BETWEEN AREA 2 AND AREA 1 FOR ROUND TRIP FARES
BOOKING CODE EXCEPTIONS FOR CARRIER: UA
IF VIA AC E- FARES VIA ATLANTIC
VIA UA M REQUIRED WITHIN AREA 1
IF VIA OS E- FARES
VIA UA M REQUIRED