Should it downshift?

Ok I was able to get something done today. I took the car out and punched it from 45MPH and yes it did downshift at that speed. Second I retrieved the serial number off the pan rail. PK432 9438 8666 6243.

Last weekend when I punched it from 60MPH and it stayed in 3rd maybe that might be normal??? I would still be curious what the serial number says about this transmission just so I might know a little more about what I have. Thanks.