I've got one of the 200 sample FH machines. Here's what I know/info I've collected about the prototypes/sample machines:
There were about 10 prototype games made. These probably have red lamp boards, maybe marked with an 'X' in their serial number, and also had an additional small blue standup target: prototype games have targets that spell S-T-E-P-S, while all other games spell S-T-E-P: the small target between the mystery mirror and the hidden hallway (so left of Rudy) was dropped.
There were also about 200 sample games built. Unlike other games where the sample run has little differences, Funhouse sample machines have some noticeable changes. This was mainly because Williams was changing from the System 11 boardset towards the new WPC boardset. Funhouse sample games have the old system 11 type of siderails, a start button that sticks out of the cabinet, it still uses the System 11 sound board (together with special power board), and most important: it has a DiamondPlated playfield. The backbox wiring is also slightly different from regular production run games: it has a lot more lamps at the top and the wires are guided different. Sample / Proto games also used the wide system 11 style of siderails that go around the flipper buttons, and the cabinet artwork is a decal. Production run Funhouses used the smaller WPC style of siderails. The cabinet artwork is screened (like on other WPC games).
Best of luck with your sale! 
San Francisco, CA
Adelaide
