For the side destos, I believe that the older, more "modernist" fonts is Helvetica (I know some of you is tempted to say Arial, but - read
these Wikipedia articles first (two links in there)) or Univers. For front destos, a few of them (such as those extra-large ones for the Volvo Olympians) that were made before SBS Transit switched to the new fonts also looks like Helvetica to me. Not sure about the oldest ones and the new fonts SBS Transit uses nowadays...
By the way, can someone snap a picture of a SBS Transit side desto using the old fonts containing small letters "A" and capital letters "Q" and "R"? These letters should help in determining whether they used Helvetica or Univers...
p.s. I'm no font expert, but I have read and researched into a bit about it before...