Do you know your grandfather's serial number? If you have or can find his serial number, then write to the national archives. Give them as much info as you have; his rank, service number, dates of service, as many units as you know that he served with, etc. In other words, the more info that you provide, the better the chance that they can help you. Try checking with a local Marine Reserve unit to ask if they can give you a web site for the 21st Marines.