Not sure which was original, but I think the metal one is preferred. Normal operation activates lower flipper first and upper when pushed all the way. That way you can cradle a ball on the lower without activating the upper.

My TZ has metal, as does my BSD, both made in '93.

2-stage flippers are correct on a TZ. Sure takes some skill to be able to use that feature fully, though!

Anybody knows why my TZ doesn't have optos for flipper switches?
It has leaf switches like pre-1993 Bally/Williams machines.
Does that mean it's an early version of TZ or someone replaced this? Works fine. No bad opto problems on that TZ though

If it's a really early TZ, possibly a sample or prototype, there will be a bunch of key differences on the playfield artwork, all of which are well documented here on Pinside.

Easiest things to look for on early/sample playfields:
Green lock insert
White clock face
Absence of "Flip Here" on the mini playfield

Other things that are anomalies for very early TZ's but not always there:
Green artwork behind the three arrows for the left ramp
Third magnet installed
Left spiral plastic
White clock face, but with "color wheel" inside the numbers(this is highly unlikely as these were on a couple prototypes only)

What's more likely is that some operator at some point needed to fix those switches and ended up replacing them with whatever he had in his parts bin at the time. Some ops are quite crafty and have even built daughter boards to make their setup work with the Fliptronics II board.