I don't own a PT145 yet (probably in about a year or so) so I haven't examined the insides of them. I don't know the mechanism by which the indicator works so I can't help you much. My best advice would be to check with Taurus. Sorry I wasn't of much help.
However, my real reason to post here is to welcome you as I see that this is your first post. This is a great board where you can find a ton of useful (and entertaining) information. I would be very surprised if someone here can't help you.