Small size FRN's are maybe a bit boring compared to a lot of other U.S. currency issues, but I find the evolution of the notes somewhat fascinating.
I base my U.S. currency type sets on the Friedberg design numbers, but I often add additional notes when I feel there is enough difference to warrant an extra example.
I have all the Friedberg design numbers for the $100 FRN's, and have just added my first "extra", a different series year for design #230. Here is what I have so far...
Friedberg Design #229: "Redeemable in gold" obligation, numeric district

Design #230: "Redeemable in gold" obligation, alpha district, series 1928A

Design #230: Same as above, series 1934

Design #231: "Legal Tender, Redeemable in lawful money" obligation

Design #231-A: "Legal tender" obligation

Design #231-B: Design change

Design #231-C: Design change
