[Fizzlemizz]

Your original code with a few print statements

Slot = {}
print("Slot =", Slot)
Slot.__index = Slot
     
setmetatable(Slot, {
        __index = Container,        -- makes the inheritance work
         __call = function (cls, ...)
        local self = setmetatable({}, cls)
        self:_init(...)
        return self
      end,
})
     
    -- This is the _init function.
function Slot:_init( bagNumber, slotIndex )
                
        --Container._init(self)               -- call the base class constructor
print("self =", self)
        self.is_a = "Slot"                   -- in the parent class, this is set to "Virtual Container"
end
     
    -- the is_a() method
function Slot:is_a()
       return self.is_a
end
    
local s = Slot(1,1)
print("Finally s =", s)
print("Value of is_a", s.is_a)

At each print (excluding the last) you will notice the the table is the same. That means that during the intitialisation of s (local s = Slot(1,1)) when the code gets too:

self.is_a = "Slot"

you're actually doing

Slot.is_a = "Slot"

In effect overwriting the original funtion declaration of:

function Slot:is_a()
       return self.is_a
end

and turning it into the string "Slot"

I think you're thinking that s should actually be a new table entry in the Slot table.