[Edit] Guilt at 01.00 hours is a terrible thing...
Complete post re-edit with pics for anyone that read the TL;DR original!!
As Inge says, the original crashbar design bolts directly to the sump at the rear. Don't use them; BMW re-designed them into the later fully-rubber mounted type because impact stresses were shattering sumps. The later crashbars are recognisable because the leftside lower mounts have a cast piece at the front and a tube at the rear, whereas the early ones have flattened ears (same as the rightside which has flattened ears on both early and late versions). Also, the early type require a triangular mounting piece on the leftside which isn't required with the later style crashbars. OEM Parts shows the early type. That's my experience: someone else may have other information.
Caveat here: it's an 8mm thread on the rubber mounts. Getting the nuts over the threads inside the crashbar mounts requires a quarter-inch drive 13mm socket because there's no room for anything else (except perhaps a box spanner). If the nut rusts on then there's a high chance that the socket will round off/shatter. I put a 20mm deep 8mm thread inside a length of 13mm hex bar (approx 30mm long) and used that instead of a nut on both lower mounts. It means I can get a standard half-inch drive socket on the end.
First pic is the leftside lower. Sorry about all the sheep and cow poo obscuring most of the engine, so I've outlined the mounting blocks. The engine shadows are hiding my extended nuts.
Second pic is the rightside just for reference. The rear rubber mount is just visible below and behind the rear crashbar nut.
I can't show the top mounts as they are inside the fairing and 01.00 hours guilt isn't
that powerful.
Just to remove any confusion on the second pic: you don't have another screw/bolt below the front mounting. On mine, the rubber block started to come adrift from the mounting bar that bolts to the sump, so I cleaned it, glued it and then put a 5mm screw through it for good measure.