Hey man I looked forever! Off the top its made after 1990 'cause its a 900. I know when the MKIII and the SL-X JCM900's were introduced but I do not know about the DR's!! (4501 is DR). But ge tthe serial number and hopefully this helps:
The Dual Reverbs were introduced in 1990 along with the MkIII's. The SL-X was introduced in 1993, replacing the MkIII. I know my MkIII is a 1990 based off the serial number. The link posted will help you.