Thought it was just the handle/ adjusters on the seat that are different? early has a bent lever (to one side)to slide back and forth and the late has a flattend handle in the middle of the seat?
73 seats will fit into 69, should be the same style of seat & runners.The late seats 75/76 on wards won't fit with out changing the runners. T25 seats won't fit but will fit 74/75 onwards drivers side only. Hope this helps.