According to Steve Ritchie (on IPDB) 10 prototypes were built, but there seems to be a number of games with prototype features making it confusing.
I have a prototype (with an "X" in the serial number) and it has different cabinet art than the production game. Your serial number seems to show a later game, but I have many of the differences you have on my game (clear beacons, "proto" stamped on the rom's, different sounds, instruction card).
Does your cabinet have the normal production art or the art that's missing the red stripes (and entirely different art on the sides of the head)? Also, what does it say on your speaker panel? Mine says "Williams Number 1 in the World". I believe the production games say "Williams Pinball #1 in the World".