Sounds like you have a Mac 128K which is branded as a 128K. These were sold alongside the 512K for a while, and if yours was made in 1984, this would make a lot of sense. I think the 128K was sold at least well into 1985.

This link might assist, and is also the Mac Registry if you wish to add yours:

http://baseportal.com/cgi-bin/baseportal.pl?htx=/Mac128/M0001_Registry

Also note, your serial number should end in M0001 for the original, or M0001P for the 128K (I think). My 512K is WP.