The 1966 through 1968 coaches had Owens Corning windows that have been obsolete for a number of years. Once in a great while, one might be uncovered in New Old Stock at a dealer; but beyond that new Corning windows are VERY scarce. The earliest of these coaches did not have frames along the perimeter of the glass while the later coaches did - - I am not certain but believe the progression was '66 no frame around perimeter of glass - - '67-'68 perimeter glass frame. Today, an RV salvage yard or E-Bay are the most likely sources of a Corning window to replace a broken or missing window. Many will use Lexan to replace a broken or missing Corning window either as a permanent repair or as a "make-do" until a real Corning unit can be found.
Inland RV has the clips, operators, and frames for some of the Corning windows listed in their on-line catalog that can be found at:
Inland RV On-Line Catalog
Good luck with your search for Corning Windows!