Must have been sweetheart deal. The lowest 30 year fixed rates, from the fifties was betweent July 1950 and February 1951, where long-term rates averaged 4.08 percent. By '53, they'd shot up to over 5%, and stayed between 5 and 6 for most of that decade. Of course, conventional then, just as it was through the eighties, required a minimum down payment of 20%. It could be that your father had a GI/Veteran Administration mortgage, as those were about, even at that time, the only ones that were assumable. Moreover, 5% down was common for those mortgages. And, it could explain that lower than market interest rate.