It seemed to be an obvious question. If there were STuG III and StuG IV German tank destroyers, where are I and II?
I learned StuG III was labelled “III” because it was a big assault and later 75mm anti-tank gun mounted on a Panzer III chassis without turret. Later they mounted the gun on a Panzer IV chassis without turret and called it StuG IV. This was done mainly because the factories producing Panzer III chassis had been bombed and the Panzer III tank was obsolete anyway; therefore restarting production was deemed too troublesome.
Omitting the turret allowed a much bigger gun but also meant you had to turn the whole vehicle to aim. But if the gun was big enough they can snipe at enemy tanks beyond their gun range. Tank destroyers were very effective when fighting on defensive.
Later they mounted even bigger 88mm gun on the Panzer V Panther and 128mm on Panzer VI Tiger chassis, in the same fashion without turret. Instead of StuG V and VI, they called them JagdPanther and JagdTiger (hunting panther and hunting tiger, respectively). These took tank-sniping, I suppose, into an art form, combining long range with tremedous punch.
They did, early on, they did mount a 75mm gun on the Panzer II chassis, and called it Marder II. The turret was only big enough to mount 20mm gun, which is measly for a tank, therefore they mounted the gun atop the hull without turret. And before that, they used captured French Lorraine artillery tractors to mount the same gun and called it Marder I. If there had been StuGs I and II, the Marders would be those. (The Panzer I hull was too small to do anything more with it.)