The bar code on the playfield is scanned at the
end of the assembly line, serial number stickers are then made from
this info as well as coin door info and the bar code number represents
the last six digits of the serial number.  Coin door info represents
the second and third digits in the serial number.

To answer the original question, look through the coin door opening
on the left side of the cabinet.  There should be a serial number
sticker there as well as smaller ones on the front of the cabinet
below the coin door, on every circuit board in the backbox, on top
of the backbox, and maybe a few more spots.

The sticker on the left side of the cabinet inside the coin door is
a duplicate of one of the stickers that was on the rear of the
cabinet.  Besides the serial number, it gives the manufacture date.
The manufacture date tells when the game was made, not the serial
number.