The Caterpillar is a Caterpillar Sixty. Here's a photo of one. http://www.flickr.co...455/5599054373/ You can juuuuuuuust make out the SIXTY on the side of the Cat's big radiator in my old photo.
Apparently the Caterpillar company was not formed until 1925, when a merger between Holt and the C.L. Best Gas Tractor Co. So this photo has to be from 1925 or later, because otherwise this tractor would say Best on it, not Caterpillar. The Sixty was a Best design. The Sixty was made until 1931, per Wikipedia.
So I'm surprised. There must have been an interim surface for the National Road laid down in 1920. I'd bet on gravel. And then in or after 1925, the brick was laid.
The eBay auction is here: http://www.ebay.com/...984.m1439.l2649 (while it lasts). In it the seller says these are from the 1930s. I'm not so sure. My gut tells me late 1920s now.
The anecdotes I've heard suggest that Illinois launched a comprehensive state-road-building program in the 1920s and it put what Indiana was doing to shame. The Terre Haute to St. Louis path of the National Road (though 1800s construction stopped in Vandalia, it had been surveyed to STL, and I'm under the impression that local communities may have built the rest of it or at least blazed the trails) was important and would likely have been among the first built under whatever IDOT was called back then.
The National Road was State Route 11 before it became US 40.
And holy frijoles, as I was looking stuff up just now I found a cache of historic IL state road maps! http://www.idaillino...collection/isl9
The 1921 map shows the NR as being mostly dirt with some "hard" sections. The 1922 map removes surface descriptions, daggone it. There's no 1923. The 1924 says the road is hard surfaced.
Soooooooooooooo this makes it all as clear as mud. The Caterpillar tractor can be no older than 1925, but the 1924 map claims a hard surface. I'm going to call these photos "ca. 1925" and move on. Close enough for me for now!!
Thank you so much for your help with this Dave. You are a treasure!