Recently purchased this unusual (as far my experience is concerned) double ended ball point pen. My question is this: what kind of cartridge could it have used? The center piece into which both pen tips screw into is only 1" long. Thus, once both tips are screwed in, there is only 1/4" remaining between the two ends (they thread in approx. 3/8" each). Since it clearly once used a red and black ink there must have been two very small cartridges....Any help would be most appreciated.