Fixing screws

I lost the single security screw (the one with a Torx head) when I went to replace the charged-up battery. The cost of a new original one is ridiculous, so I replaced it with an M3 x 5mm philips screw. I found out that the other (Mk2?) version, with 2 screws holding the bell onto the wall/door bracket is also M3 but longer. I used 2, M3 x 10mm philips screws, as the bell had been delivered sans screws.