You could use the method place in order to adjust the label correspoding to the element 1.

The code:

    # Crear frame and pack it
    self.test = tk.Frame(self, width=60, height=40, bg=COLOUR) 
    self.test.pack_propagate(False)
    self.test.pack(pady=20, padx=20)

    # Crear label for "A"
    self.testletter = tk.Label(self.test, bg=COLOUR, text="A", font=("Helvetica", 16, "bold"))
    self.testletter.place(x=5, y=5)

    # Crear a label for "1"
    self.testsubscript = tk.Label(self.test, bg=COLOUR, text="1", font=("Helvetica", 8, "bold"))
    
    # Ajust "1" below "A"
    self.testsubscript.place(x=23, y=20)`