That, I believe, is Cadillac part number 147 4733 (evaporator drain tube) and fits 1959 through 1964 CC & 75 series, and 1965 - 1968 series 75 cars. Mounts on the rear of the evaporator, and cost $.70 in 1968. I don't have a part number for any of the other divisions, so it is going to be more difficult to locate. With 5 divisions... Chevrolet, Oldsmobile, Pontiac, Buick, & GMC, they had 5 different part numbers for the same parts. If my information is correct, it will measure 4 3/4" long. The earliest reference that I can find for this part is on a 1955 Chevrolet air conditioned evaporator box.. Part # 3722674.
I temporarly deleted this post while I did some further research to verify the above info. Unfortunately, I wasn't able to locate this part through any of my NOS parts suppliers. Still trying to find the Buick, Pontiac, & Oldsmobile part numbers.