You are correct about the rate being 15c/half ounce. Additionally that rate was effective 10/1/46 to 7/1/61 so this letter wasn't mailed right at the beginning or end of a rate. I have seen letters posted beginning or ending of rate where the postal clerk apparently forgot to take the change into account. That doesn't appear to be the case here. I think more likely is that with all those stamps the clerk simply calculated the postage on the letter incorrectly. When all else fails the simplest answer is typically the correct answer.