We had one when I was very young. The sledge ran on two timber runners about 200 x 50mm with steel wearing strips.There was a platform behind baler about 1metre square where the stacker stood. Behind this was a tilting floor which could take two bales laid fore and aft on edge. Spikes fitted to the runners projected through this floor and held the bales in place.
9 bales were then stacked (2,2,2,2,1)and when the last one was in place a bolt on the floor was kicked to allow the floor to tip gently backwards, lifting the bales off the fixed spikes. The theory was that the 'stook' would slide gracefully off and remain standing! Theory and practice did not always agree.
The floor would return to horizontal and the bolt locked it in place ready for the next bale. This was soon superseded by the 'bulk' collectors and then 'square eights'.