Earliest Homeline models used a back fed main for 125 amp and less and a 4 pole looking device like some other brands use for 150 -225 amp mains.
It wasn't very long though then they went to same main breakers as the QO line uses - QOM for up to 125 and QOM2 150 - 225 (think there is QOM2 frame size for 100 and 125 amp also) series breakers, if you have one of those earlier Homline panels the QOM series won't fit them though.