I've never been given any really old bills in change until the other day. I went thru macd's for lunch and she gave me four dollar bills and some coin in change from a Ten. These two were mixed in 1926 and 1935.
Check that 1926 bill again. US paper money was reduced in size in 1929. Since they are larger, pre-1929 bills would not be confused for current money by a McD's cashier, as it wouldn't fit in her register drawer. If your '26 bill is correct, it should be larger than the '35 bill. If it's the same size, then it might be a '29.
EDIT: Looking at your "1926 bill", I clearly see a "Series 1928A" written to the left above the blue seal. That means it's a 1929.
Unlike coins which are dated for the year of release into circulation, bills are dated for when the design was created. If the design doesn't change from year to year, neither does the date. If the design changes more than once during a year, then they add letters after the year, like A or B. For example, $1 bills dated 2003 were made in 2003, but then one of the signatures changed mid-year and bills after that point were designated 2003A, and were printed through 2005. The signature changed again in 2006, and all $1 minted from then to now bear that date.
wow! gotta love it when people don't know what they have and just spend it. I have one silver cert. that my girlfriend got in change, she thought it was a fake bill and was going to go back in and complain, but I managed to stop her.