Peterson-Beemer list shows 310001-025 UDLX
310026-500 UTS (1938)
310626-646 UTS (1938)
310647-950 UTS (1939)
310951-311150 first UTU (1939)
311151-350 UTS (1939)
311351-550 UTU (1939)
311551-312450 UTS (1940)
Lloyd McCray's list is consistent with respect to UDLX but shows other tractors in the early time frame only as U and does not distinguish UTS vs UTU. McCray's list also show engine s/n 1938 KEA 531754-532842
1937 KEC & KED 540001-540025
1938 KEC & KED 540026-540575
1939 KEC 540701-542553
No indication of any engines between 532842 and 540001 - so as I said it would be very interesting to know any prefix or suffix associated with a 539xxx motor.
Brian Rukes s/n list for 1938 is consistent with other lists showing 310510 should be a UDLX - so best I could say is it may have been a prototype for the narrow front UTU configuration.
If you come across any more 310xxx narrow front UTU, please note S/N and Engine S/N and post here.