Early games with serial numbers below 18900 had yellow text on the bezels, G05-801 monitors, a copyright sticker on the picture tube and round owl eye coin doors.
Later games with serial numbers greater than 18900 had white text, either 19V2000 or G05-802 monitors, and typical slot coin doors.
Internally, the early games also had a different harness to accommodate the monitor connection. You'll also notice an evolution in control panel construction, with early games containing a plywood control panel assembly under the usual metal panel. The leaf switches on very early games are screwed to this plywood assembly.
Later models used a leaf inclusive button assembly. I'm unsure if the change in control panels coincides with the major changes above serial number 18,900.