From left to right, empty Platinum cartridge, current Platinum piston converter, modified Platinum converter, and old type Platinum converter for pocket pens.
— Use Platinum proprietary cartridges, either new or by refilling them.
— Use an adapter to use short international cartridges or short converters (squeezer type).
— Adapt the current Platinum piston converter to fit inside the pocket pen, as was already described on these chronicles.
— Finally, find an old Platinum converter.
The following table summarizes the cost of these options:
Prices in yen without taxes. Those of the old Platinum converter and the adapter for international cartridges are taken from the Internet. These prices can change a lot among sellers.
The old converter is hard to find, expensive and poorly made. But it holds more ink that the current converter, especially when the later is modified to fit in a pocket pen.
My choice is pretty straightforward—I refill cartridges with a syringe.