The box is definitely authentic, and in really great shape. I have the same vintage unit, and my literature is marked 3-48 (date). Does this unit have the slide stop stamped "2" on the back? Hard to tell from the pics if it is there. That looks to be a $500-$600 value. Hope you did well on the purchase.
Seems like there are quite a few Colt .22 Conversion Units being sold, or at least getting listed on eBay & GB. I have the same vintage Unit no box and the incorrect slide stop. Used it to assemble a Service Model clone.