What is the difference between between the last digit C and D on gas valve part number

The manufacturer isn't providing a clear indication as to the purpose of the last letter in the part number. We have seen, at times, the last letter in the part number may be a generational change. In reviewing the difference between the C and D, the parts appear to be unique and it's unclear if they are interchangeable. I wish we could provide more details as to the difference. Call Rheem support directly and they can provide the proper replacement gas valve for your water heater.  https://www.rheem.com/contact