The STOPPIN1 is used with older (-M and early -S) die sets
which have a slotted head ejection pin in the point form
and jacket end rounding die. The standard pin which comes
with the CSP-1 S-Press is the STOPPIN2, a long pin which
passes completely through a hole in the newer punch heads.
But the older dies can be used in the latest press, with
the addition of this part.
The length of the pin, measured from the tip to the point where it enters the knurled head, is 1.065 inches (anything in the 1.05 to 1.08 range is fine, however). The longer STOPPIN2 is about twice that length, at 2.050 inches. The pin diameters of both are 0.238 inches.
The short pin can be used in the S-press, with slotted head ejection pins. The long pin, however, will not work in the early presses such as the Series II, because the ram is not machined with a slot all the way through it with a support hole in the rear of the frame, as is the current press design. Therefore the long pin would be sticking out of the front of the press about half its length, and the pointed end used to help align and guide the long pin would not fit into the notch in the early slotted head punch (used for point form dies and jacket maker dies only).
The current ejection pins for the -S press have a hole all the way through the head, not a notch. Also, the diameter of the punch head is larger to allow for this .238+ diameter hole, so it does not fit down into the smaller hole earlier press rams. The older type M dies used a smaller diameter ejection punch head with a slot or notch in one side, about half way through the diameter. The squared off end of the short punch fit into this notch to hold the pin. The older -M punch will fit in the latest -S press, so you can continue to use early dies provided you also obtain and use the short stop pin with them. A later -S type die and punch will not work in the earlier presses which were made for smaller diameter punch heads. Basically, there is no drawback to upgrading to a later press, because it works with every sort of punch and die of the -M or -S versions, with or without a hole all the way through, provided you add a short stop pin (the long one comes with the press).