Daniel : According to my reference material : Greenbergs Guide to Lionel Trains, O gauge, Volume 1: 1915-1928 the 810-820 series cars did at first have a short wheelbase In 1926 they were made with a distance of 4 7/8 " from truck center to truck center and in 1927 through 1934 this was changed to 5 1/2 ". Both frames were flat on the bottom. The reference does not provide any reason for the change. So this does confirm your finding that this variation was indeed produced by Lionel for the 1926 year. The Greenberg reference lists the same value, Good $30 and Excellent $100 for both the 1926 short wheelbase version and the 1927 -30 long wheelbase version. Obviously since the short wheelbase version was only produced for one year, it would be more scarce.
Best wishes...great find
Don
