One of a kind?

I hate to use the word "rare", especially on items that were mass produced. Condition, attrition, and others factors can, however, make even those items rare, over the years.
Occasionally, one finds an "anomally", such as this. The locomotive is nothing special, I have others, like it. However, that yellow box, factory lettered for this item, may well be the only one. Athearn ended the "yellow box era" in 1966, with the introduction of several new cars and locomotives, including the SDP 40. The box color became the very familiar blue box, that we grew up with. At least one yellow box, however, snuck through, and this is the result, a rare, or even one of a kind, combination.