Is the triangular mass a rubber pad? I am guessing it is part of a sidecar. The grommets on the floor on the right would attach to the bike higher up. The heavy brackets, those 4 holes attach to bottom frame of the side car. The grommets touching the floor on the left side would hold struts extending to the lower part of the bike. The remaining grommets attach to the body of the sidecar. I am making this sound plausible. I am admittedly full of crap, if anyone has a better guess, let us know. I googled 20 or 30 pictures of sidecars and their attachments. I found nothing that looks like yours, albeit they all looked different from each other too. I know nothing!