If the Plus is beige, it was made in 1986-very early 1987. All Pluses produced afterward were platinum. Later production Pluses (late 1988 or so onward) have simply "Macintosh Plus" on the rear bucket, not "Macintosh Plus 1MB".

Some floppy drives used similar date stamping, although this didn't seem to start until around 1988. If the keyboard and mouse are original, they may also cue you in a bit.