Sure. You could order any of the CPUs with the D300 GPUs. It's totally possible someone could have installed the 6-core CPU in it though. Apple should be able to tell you the original specs for the machine based on the serial number.
I don’t know of anywhere you could enter the serial number online to get the specs. There are some serial decoding websites, but they don’t give reliable info and it’s probably best to avoid sharing your serial number on random websites. I would start a chat with Apple support and ask if they could look that up for you.