I recently purchased a 2003 R50 Hatchback with VIN id: WMWRC310x3T, whereas you know, RC310 digits are the vehicle equipment identifier. I've looked and looked but the closest I have uncovered is the RC334 designation (indicating airbags, left & right). Could RC310 be a model built for Spain?

The RC31 portion of the VIN identifies the MINI as a European Cooper ("RC3") with left-hand drive ("1").

You can use the last 7 of VIN to find out the build specification: https://www.mdecoder.com/