I recently acquired this prototype BSD. Pics below.
Its actually my third BSD, the previous ones were obviously not as nice and unique as this one. Amazing game, definitely a keeper for me!

I'm looking for some stories about this game. Maybe someone can share some stories about how prototype production went at the time, and maybe what they were planning to do on this game. There are many loose connectors and cut wires which hint about features that never made it to production.

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Proto number 4
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SUN easter egg works

I did not notice any differences in the software.
What software version is this?

Some sound effects are different. Especially speech on right ramp shots have some special effect which is not in production.
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There is some strip of metal welded to the castle ramp. Different text on playfield.
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Green targets.
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Asylum scores 25M
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Different VIDEO colors
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No metal guard on ramp, instead some posts. You can see through the plastic.
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Notes on the lamp board.
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Install 3 balls. The software requires 4.
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Red prototype board for game-specific features.
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(Now I'm going to put some mylar and metal protectors on it)

Sounds are on different rom chips than the code itself so I think they are released separately.

Yes it is Very well done!

Indeed, the software feels the same as the production version. Asylum scores 5mil and such. The targets don't do anything with 'windows' so castle locks are just as in the production version.

I also noticed the game has trouble keeping track of balls, also sometimes it forgets to load the mist ball after I cleared out all the balls. No problem though, if you know about it you can easily help the game.

I've heard rumors that the houses would be glow-in-the-dark but I have not been able to verify that yet.

Apparently this game was donated to IFPA by Williams
https://www.ifpapinball.com/wp/wp-content/uploads/2013/07/ifpa3.pdf

Possibly from the same IFPA event

BSD is the number one pin for me and having a high end restored, unique prototype pin is like hitting the jackpot. Its not going to get any better than this in my opinion. It scores and plays the same as the production model. Maybe the production model has a better playfield, but its mainly text and colors. The text and colors are not bad on the prototype, they are just different. The nice thing is that I get to explain almost every difference on this game so it always makes a nice conversation.

Going to take some time as I need to set up good audio. The effects are subtle.