Hey Simon:
The '87 means it was built in 1987. The S means it is a spoiler, and the 4454 means it is the 4,454th instrument ever built by Alembic. My '83 Electrum guitar is #2780 and an '85 Electrum in the Showcase thread is #3796, so 4454 for an '87 sounds about right to me. My Electrum also has the serial # on the 24th fret, as did most Alembics of that era. When I brought mine to the factory, Mica copied the number into the electronics cavity.
Usually Valentino or Mica will respond with build info on this thread within a few days, though they do have other work to do.
Bill, tgo