It is interesting to me that some of the build sheet dates towards the end of the month have a door sticker with the following month. 71 GMC on this post is one example.
1Z653410 = 05/14/71 (post 440)
9Z870618 = 04/25/69 (post 440)
Z120204 = 03/29/71 (post 440)
ZZ54546 = 11/08/69 (post 440)
8J119920 = 12/67 (post 440)