Haven't finished reading the thread, so I don't know if you've been answered yet - if so, please forgive the duplication
I ordered this set and just received it this week. It's the 3-piece set - I call them "Shorty", "Regular" and "Helga"
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"Helga" is in the middle. They're a top-fill and don't come with bases. I am currently running "Shorty" on one of my mods. I kinda like the top-fill aspect (so I don't have to keep threading and unthreading the topper from my mod. I haven't tried out "Helga" yet. I figure I'll use that one when I'm going to be somewhere I can't get my hands on any refills for quite some time
In comparison, take a "Regular" STM and add again the top metal height to get "Helga". Take away the top metal height, and you basically have "Shorty"