The parts catalog says indeed there are a few types used (the part has predecessors). But I think they were all torx bolts fitted in the z4m and that only the m3 recieved allen heads.I'm not sure but I think pre 2007 z4m s54 used a different kind of bolt, it was superseded with a torx head style, obviosuly I can't tell from that phot which type those are.
The only thing I can see is that the previous type was 2mm shorter. It's not really clear if any were fitted in the z4m; the latest version was used from 4-2006 upwards according to the parts catalog, but old stock might have been present.
BTW the snapping of the bolts is I think not an part issue, but probably more a fitting issue.
I can't find the exact procedure of fitting those bolts in TIS. Beisan etc specifically says to thoroughly clean with brake cleaner and apply locktite.
That might be different from factory.
The bolts wear because they come loose and the hub they hold in place starts to rattle against the side of the bolts, slowly eating its way through them.