Yes, this is normal. I was a bit astonished too, when I got my A1 as it differs from the A1 Mini here (which has 4 screws).
If you run Manual Bed Tramming, you will notice that this corner is left out as there is no screw.
So, 3 screws seem to be sufficient to level the bed.
The debris you mentioned can only be left over from manufacturing process as there is no way it would get there from the printer.