Most tube mechs are bottom fired.
Two top fired mechs are the Poldiac and the 69.
Most mechs come with 510 only threading. Few come with 510/eGo threading.
Alot of mechs, especial bottom fired mechs, come with adjustable top center pin.
Some (few mechs) are telescopic.
Popular mechs are available in a variety of materials: steel, brass, or copper.
Some mechs have silver or gold plated contacts.
Another thing is max battery length; I bought a mod that will accept a regular 18650 and a kick module too; with some mechs, if you want to use a kick you have to buy a shorter battery size to make it fit.