Singer treadle irons come in two sizes and of the smaller size, there are two styles. All of them will work with the any model 66. The wooden pitman indicates that it is a very old set, I think Singer went to metal around 1895 or so. So yes, the pitman only helps indicate vintage.
One thing you need to be concerned about is the flywheel. You need the larger spoked flywheel for it to work well as a treadle machine.