Today I got myself a used Legend (known as Lenovo of today) keyboard and brushed it up, just to find that it has some interesting stains:

Since it's widely known that every stained keyboard might tell a story of its former owner, we could make a speculation of what it was mainly used for back in the day.
However, every single (limited) educated guess of mine doesn't make perfect sense:

1) Although the Z, X and Ctrl indicate an identity of production machine, it's likely not to be that way since S seemed to be rarely used;
2) The only possible usage of Tab key apart from making spreadsheet (in my knowledge) would be gaming, but neither WASD or UDLR seemed to be heavily used;
3) The yellowed Windows key doesn't have too much useful combinations with the rest ones.

Any ideas what this keyboard could possibly be used back in its days?

PS: This is a keyboard from year 1998~2000.