Great question. It could be something as simple as the backplate number was made larger or smaller. Doesn't always have to be a drastic change. I'm curious though too... what was different?
I'm a novice here when it comes to large denominations. I own zero. I see no difference, but the internet says, "For the Series of 1934A $500 Federal Reserve Notes, the “mule” designation is specifically tied to the plate number size transition that happened during the change from Series 1934 to 1934A." I THINK it is the front plate number that was supposed to increase in size/font but didn't, someone fact check me. I didn't see a noticable change in the back plate number, but it could be that too. Large = Normal Small = Mule
-Thanks! It is funny you mentioned Polar Currency YT as I just finished reading this thread on Numista & checked his YT channel out. (A good thing to do when you're not buying- check out online resources). It was very interesting & entertaining. The BoC series are fun b/c they have limited runs even now. You can actually acquire some inexpensive tough notes in the polymer series (& then go back to Upgraded Journey & Original Journey). I created my NC Home page centred around World & Canadian Banknotes back in 2019. If you take the link to each series, you'll see how I have identified the best (tough/collectible) prefixes to look out for. Some of the change-overs have less than 1 million released.