I think you meant that your later plough was a TS39 (TS59 is a mounted plough). This was definitely offered with YLs and went into production in 1940. If the frame width is fixed at 12" your assertion that YL183s were used at the outset would appear to be correct.I run several 6 furrow Hexatracs. These were definitely offered with a YL option, my early one (pre-war) is adjustable but appears to have always been set at 10" and is on 165's and one of the later ones is a TS59 with fixed frame at 12" and has 183's. I was always told that this later plough was one of a pair that were the last ones made by Ransomes in the late 60's to go behind a TD20 or similar. Obviously the boards could have been renewed during its lifetime, but I somehow doubt that a plough designed to be run at 12" wide would have been fitted with 165's. My other TS59 had deep digger boards, and I swapped the frogs to YL, just bolted straight on with no alterations. My point being, that trailer ploughs were still being made very late on and were being made to take 183's. Not sure about earlier, smaller ploughs, but logically, since Ransomes made such a wide variety of ploughs for different markets and customers, I think we can be pretty sure that there will have been customers specifying 183's on a wide range of ploughs as soon as they were available. Incidentally, if anyone wants them I have four brand new original 165 boards, surplus to requirements.