Is there a way to tell how old my fuzes are?

Thanks guys.  I knew about the firmware, but what with all the updates I thought it would be hard to nail down.  On the oldest one I bought brand new BI0910BMXK  an 8gb it has firmware ver.2  I wonder if I had put on wrong firmware update by mistake at some point?  It’s well used, well loved, but the audio jack loose.  Funnny thing is it has the 2nd lowest number on the back…  I was hoping the numbers on back would go up as they got newer?  I don’t know why Sansa couldn’t tell me if this was the case, or not.  It’s not like any of them are under warrent anymore.