The man I would have to ask is no longer around. But I am guessing 54 would be in the ballpark if that was when a cornhead and 45 combination could first be purchased together. I was born in 54 and I also remember dad saying 54 was a terrible drought here in SE Kansas. 

I just remember as a kid him telling me and my brothers the story of how he paid for the combine with the income he got from custom shelling corn the first fall that he had it and there were almost no other SP combines around with corn heads.

Keep in mind back then the areas farmers traveled were pretty tight. It was a major trip if you got out of the two counties where we are on a county line. So other areas probably were more advanced than his local area. Sounds like the 55 had corn heads available several years earlier but that would have been a big combine for this area in that time frame. A lot of farms were no bigger than a 160 quarter section.