As US 20 gets it's final divided segment in Western Iowa, I noticed this in Sac County- a "Historic US 20" sign. D36 replaced 20 west of Fort Dodge to IA 471.
I'm guessing that Rockwell City and Sac City were getting hammered by lack of 20 traffic that they had to take matters into their own hands. I don't see any other counties with these signs so far, but many appear on the old US 6 route.
It's probably from that guy who's posting them randomly including on modern US 20.
There is a project underway to mark the old alignment of US-20 in much the same way as old US-6 is. I'm not sure the former has quite the same support and funding as the latter does/did, though.
http://www.kwwl.com/story/36568671/2017/10/11/turning-an-old-highway-into-the-new-route-66
The signing of Historic US 20 across the country is the result of Bryan Farr and the Historic US 20 Route Association (http://www.historicus20.com/index.html). He's been traveling across the country over the last several years meeting with local officials, chamber of commerce people, etc. in an effort to bring more attention to the old route. Part of his work is to get more former alignments signed in the field with historic route markers.
I saw a Historic 20 in Mentor, OH when I was up there for the bridge meet. It was rather small: I'm thinking 8x12 inches.
(https://c2.staticflickr.com/2/1977/43888992420_266ca22ceb_z_d.jpg)