From my understanding a mule is a note that has a front and back plate from two different sets. The number is usually smaller on one side then the other. That is the plate number. I could be wrong but this is what I found so far. So the front of the note has the 1934A Large 2 plate no and the back has the small 2 plate no.
Not a bad note from the photos. Excellent centering. I probably would have tried to get one with the EPQ/PPQ designation, but that would have cost you more also. Label also doesn't mention the staple holes so I guess the grading company missed it or forgot to note it. As @edteach mentioned, it's when one of the plates used is from a different series. This creates a "mismatch" and usually indicates that the note was printed in one of the earlier print runs of its series since one of the plates from the previous series stayed in use to start the next series.
@edteach and @Endeavor described mules well. Many of the early $1 FRN,s printed in the 60's had FRN front plates and Silver Certificate back plates.
The most he could have gotten was five hundred silver dollars, containing around 385 oz of silver. About $7000 today. Or he could have used three $500 bills to buy two Apple I computers in 1976, with enough gas money to drive to Palo Alto from Chicago to pick them up and take them home. They'd be worth at least $150,000 today. Each.
The melt value might be $7000 but the per coin value is higher than that. 500 silver dollars is going to be in the $10,000 area. It's all going to be Peace dollars and Morgans, some in MS condition.
This is actually a misconception. In the small-size era, there's no such thing as a "Silver Certificate back plate"; all back plates are uniform and therefore interchangeable between types. Whenever a single denomination was being printed in multiple types, it's not hard to find notes of different types printed from the same back plate. Most of the time, nobody pays any attention to this distinction; nobody much cares if you pair up a 1928 $5 FRN and a 1929 $5 NBN with the same back plate. But for no apparent reason, some people will pay strong premiums for some 1963 $1 FRNs that share back plates with 1957B $1 SCs. Even more oddly, it only goes in one direction--I've never seen anyone pay a premium for 1957B $1 SCs that share back plates with 1963 $1 FRNs, though they're theoretically just as interesting (or just as uninteresting).