As I went to my bank box last week, I came across a series 2006 $5 FRN that was the old style, and I thought nothing of it until last night when I saw that the new purple notes were also series 2006 (I thought they were going to be series 2008). So I looked on the BEP site today and sure enough, there are two series 2006 $5 note types, one with the old design and one with the new design. Who would have guessed?

Like the 1935 G no mottos and with mottos - a variety within a series.

Yeah, but this is a little more than adding mottos! I always thought the policy was to change series when there was a major design change, guess it's the exceptions that make currency interesting!

I'm not much into currency, but I thought the controlling feature was signatures. Has there ever been a major design change without a change in signatures before?

Yes. Two that come to mind right away: 1935D (wide margins vs narrow margins on reverse) and 1935G (with In God We Trust vs without).

I loooove currency (and have spent more money on it than coins probably) but I can never remember when they change the series year versus series letter. The only signatures I can ever remember are Speelman-White, Woods-White, and Woods-Tate. But thats because those are the only ways to differentiate between any of the 1923 $1 notes!! No As or Bs there!

Over the last few decades the practice was new year with change in Treasury Secretary, add a letter with change in Treasurer. With major design changes, they's always changed the year, i.e. the series 1995 $20 old style vs. series 1996 big head.