Property name "class" is used internaly by engine and is created/added to sbx, when model is drag&drop to scene, when exist ini file with same name as model, then is added your property names from ini file to SBX too.
From my point of view, for you is better, create property name like "gameclass" or "classname" to ini file, when you don't need change value.
When you want use lua, then add to section
function class:InitDialog(grid)
self.super:InitDialog(grid)
group=grid:AddGroup("MyProperties")
group:AddProperty("gameclass",PROPERTY_STRING,"OBSTACLE","My game class"
end
Note: this code is only example, not tested.