The coupling rods on the Oxford Model are, correctly, mounted outboard of the connecting rods.
Beattie Well tanks (prototype and Kernow's model) are also like it as is the preserved T3 4-4-0, so it was probably usual LSWR practice on outside cylinder locos built prior to the adoption of Walschaerts valve gear on Drummond and Urie 4-6-0s.
It presumably minimises loco width by allowing the cylinders to be mounted closer together but I don't know if it was a done by other railways (possibly the GER where Mr Adams worked before his LSWR days?); maybe others can enlighten us
I would have thought the connecting rods being inside the coupling rods would be that many loco builders in the earlier years were still building single drivers, where the connecting rods would be on the face of the wheel. If you use the same cylinder castings which I would expect happened then the piston centres would be on the same centre line, so you can't move the centre line out with out using new castings, so the coupling rod is added to the outside.
David