Took a pic of my machine and when I looked closer, this is what I saw ...

Any ideas?

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Cut from ipdb regarding another Williams game called TZ. This happens to be from Williams WH20. Guess anybody with a TZ or WH20 and a black light can check their playfields and report back. Bet it surprises a few people.
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In this listing is an image of a playfield under blacklighting, revealing a serial number. We asked Pat Lawlor why the manufacturer did this. He replies:

The reason for the serial numbers seems strange by today's standards. In 1992, Williams' agreement with its distributors gave them exclusive sales right to a territory. They did not have the right to sell outside of their territory. But some were doing just that. In order to track the game's origin from a distributor into the wild, the blacklight number was added. Williams knew where the game was originally shipped. If the game turned up outside of that territory, it was assumed that the distributor had "Bootlegged" the game outside of his territory. Then it was up to management to decide what penalty to incur on the offender. Remember, this was during a time when it was PROFITABLE to sell and operate pinball machines. This practice would be laughable today because ANY sale of a pinball machine is a good sale.

Yep here's BSD.

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Center of playfield and above flippers on both of these. Probably so you could easily see it, and without opening pin.

Had high expectations for Predator, but we know how that one ended. Held out hope for Matrix but that opportunity passed. Alien would be ideal title but nobody has picked up on the hints. Who will be first the manufacturer to heavily use uv reactive paint? I might not live long enough to see it. Sigh...