Here is a drawing attached. The Tab is a stopper for the defrost damper door. I guess so the damper doesn't hit the squirrel cage.
The defrost in these old heaters are not high tech. It seems to me that when the damper door is open for defrost it is designed to split the floor air and the defrost air.
It would be nice to direct all the air to the defrost tubes but you would some how have to close off the floor air output..
