You are ultimately responsible for asserting your contractual rights. Your stock options had an expiry and you did not exercise them in time. The letter you received notifying that you had 15,625 vested options was not an award in of itself, it was only a courtesy notifying you that you had vested options to exercise before they expired. Even if due to ambiguous wording it could be argued that 25,000 options had vested at the time instead of 15,625, that was only relevant until the options expired. You needed to assert your rights to the [additional 9,375] vested options before they expired. So, this issue died in 1996.

I had a similar experience although over a shorter time horizon. I was in a dispute with a corporation which prompted me to pore over every word in every previously signed agreement. I discovered, due to an obvious typo in a stock option agreement, more options had vested than had been intended. After some pushback, they eventually relented and awarded me the options.

Given the amount of money involved, it was worth engaging lawyers to see if NVIDIA would pay you some money to save the hassle of dealing with it, but there is and was zero prospect of this ever being awarded in your favor by a court since the options expired.