When big bird and i set the timing on my 75 back in april we couldn't find the mark at first then we found TWO!! I can't quite recall how we worked it but it took three of us a few hours to work out what was going on.... I think i remember one mark being on the underside when the other was set in the correct position. The timing wasn't bad - we were just checking it and we came across the strange marks... we guessed the guy who rebuilt the engine must have used a wheel from a buick or something... you might ask big bird for more info...