the picture of the dismantled frame is in Sheldons EMU.......as a 1954 Excelsior Universal..............looking at the pic,I suspect the spring seat is threaded to the rod,and possibly there is a locknut.................however ,from a constructional point of view ,it would be also possible to make the rod two diameter ,and have the spring seat retained by the step between the two diameters......this method needs no thread,but has certain disadvantages...............summing up the evidence..........i would expect the central rod is "jumped up" and the increased diameter threaded for the spring seat.......the spring seat is clearly visible as a discrete unit ,secured to the central rod .