That's the Uni Style Fit gel ink cartridge, with tape around it to work smoothly in the 701. You can move the "tape stop" closer to or farther from the tip, depending on how smooth you want the click action to be.
This simple method works on a lot of pens. And with all the cool kid multi-pen refills: Coleto, Sliccies, Style-Fit. It's actually a problem, how many different barrels you can do this to. I ended up in this ridiculous spiral, grabbing any decent-to-nice barrel from my drawers and doing some test drives. A few of them weren't really worth modding in the first place, like this random Staples executive-esque pen. It's not terrible, but having a solid insert can really only elevate it but so much.
Some of these attempts at least turned complete disasters into decent pens to keep around. The Pentel Client is actually a solid barrel, both in design and construction. It's not going to win awards, but it's a good choice around the office; it looks classy but you don't mind if the boss picks it up. Unfortunately, the three of them I had lying around were filled with the worst Parker-esque ballpoint refill. It was just garbage. So having reputable refills really flipped these pens around.
And finally, there were some barrels that just shined. The Zebra Expandz is a cool pen by default. The same F refill as the 701 (and 301, 402, Spiral, etc...) is in there, being quite so average. The rest of the pen is solid (heavy enough), sharp lookin', and a great mini-pen. But when I tossed a mandarin orange Style Fit refill, it became my secret garden. Pen.