Shiney Side down
I reckon you are right and if you look hard enough most of the other explanations here actually go back to the latin "omnibus", or "for all" origin... road buses, Databuses, or whatever, they are all common carrier,i.e. "for all".
Right, lesson over, now as my dear old Latin teacher would have said "write it out a hundred times before dawn or I'll cut your ****** off"..( or was it John Cleese?)