The top cap on V3 is removable just like the V5 on all V3s made since late last summer. There are other upgrades to V3 that come with the removable top cap including the one-piece stainless steel 510/eGo threading and new firmware. If the sigelei Zmax is not telescopic it is a Mini or a V2. Both are still in production.
Here is how to tell the difference. The new production V3 (and V5) have a steel flange visible between the main body and the top cap. My new V3 purchased in November 2013 is shown on top. My first generation V3 purchased in April 2013 is on the bottom. The screen on that one was slightly crooked from day one. New V3s and V5s made since last fall also have the sigelei logo laser etched on the tube below the fire button and may have CE etched on the battery cap. The very latest V3s and V5s have metal fire buttons. My new production V3 was made just before the laser etched logos came out.
I just got a V5 a few days ago, and LOVE it. I must have an older version even though the site I bought it from said it was the latest, since I don't have the etched logo on the tube.