You should contact the Floyd County Museum for manuals and pictures, Ph (641) 228-1099.
I have several 1930 models, this was a unique year, right after the merger, and new things were being tried.
You are correct about the guage holes in the rear fuel tank support assembly. On the Hart-Parr Oliver Rowcrop tractor there was only 1 guage, the oil pressure guage, and it used part number CS-831 until SN 102930. SN 102931 and up used part number CS-831A, which had the second hole. There was part number changes on the fenders, and platforms, the first change was at SN 102135, then again at around SN 103300, but it could have been SN 103313, there is a small hole in that data for some reason. The fenders probably were the same or very similar, probably changes were made to the supports once the brakes were changed, that would be my guess. But since all my Hart-Parr Oliver tractors are 1930's, I don't have a 1931 to compare to.
Your 1930 tractor was produced as a Hart-Parr Oliver Rowcrop Tractor, marketed and sold under that model name, whereas your 1931 would have been produced, marketed and sold as an 18-27, SN 102649 thru 103300 would have been equipt with a single front wheel, then from 103301 on would have gotten dual front wheels, and some other changes. If you order the Manual of Instructions &amp; Parts List, it will give the parts changes by serial number.
I hope this helps you some!!!
Andy