The the 780 was not available with a hay header

You may be right.  The picture on the calendar may be a 215A with an auger platform. The 780 was also made for hay.  The 70 hay conditioner went on the 780, 800, and the 830 windrowers; the start year for manufacture of it was 1967.  The 78 Draper platform was made for the 780 and 800 windrower.  They came in the size of 9 ft to 21 ft. There was no auger platform available for the 780 or the 800 windrower.  If you wanted an auger platform for hay you would have to go with the 830 windrower.