I have tried several different types of these magnetic connectors and have found that there are at least four types (although there may be more). Let's call them type M, N, O and P. They are only compatible with each other within one type. This means that you can connect them together within, for example, the M group, but you cannot connect a connector from the M group to a connector from the N or O or P group. The P type connector has the strongest magnet by far, followed by the O type, then the M type is a little weaker than the O but not by much, and the N group offers the weakest magnetic connection. The M1, M2 and M4 connectors seem to be the most durable because their housings are all metal. The M1 connector is exactly the same product as the M2 connector, except that it is sold without the brand logo and box, so you can buy it cheaper.