Personally I'd quit worrying about it and just use it until it dies. Even the units that were "repaired" may fail; Nvidia said only "some" 8600M chips were flawed, but there are well-founded allegations that all of them are vulnerable and there are *many* stories out there about people who got replacement motherboards only to have those die as well. Really, what good would knowing do you anyway? Apple isn't going to fix it for free, and if you buy a replacement motherboard on eBay there's a good chance it has the same "ticking clock" on it. If the chance it's going to blow up really bothers you there's always the option of selling the machine to some sucker for a few clams profit and putting the money towards something else.

(Full disclosure: I actually have *two* of these machines, an Apple Refurb-store-sourced 15" model and a 17" model just like yours that was mine to keep after the company changed how it handled machine refreshes. Both still work fine, both might go "poof" at any moment. Maybe the only reason they haven't *poofed* yet is neither has been used much the last few years. The one thing I can't seem to find is any statistics saying what the long term failure rate of those chips actually is. Nvidia issued the recall because they were failing at a "higher than normal" rate, but it could well be that any chip that's survived this long is already past the threshold where it's any more likely to fail than any other similar semiconductor product of similar age, IE, the ones that were prone to blowing up "early" are already weeded out. Nobody expects a laptop to last more than four or five years anyway now so whatever mileage you get out of it at this point is a bonus.)