The rev. 503 comes with the BIOS version 0042. Intel fixed overheating issues and some default options were changed.
More details in the document: PCN 114477-01
The rev. 503 came with 0042, however I was using 0045 on the 502, and upgraded the 503 to 0045 as well (although I tried it without upgrading on the stock 0042 firmware as well).