1st Edition vs Shadowless vs Unlimited Base Set — how do I tell, and does it change grading?

Three print runs, in order: 1st Edition, Shadowless, Unlimited.

1st Edition: a black 1st Edition stamp on the left below the art box. These are also shadowless in layout.

Shadowless: no stamp, but the same layout as 1st Edition: no drop shadow on the right and bottom of the art box, and thinner, lighter HP text. Hold one next to an Unlimited copy and the missing shadow jumps out.

Unlimited: the common one, with a visible drop shadow on the art box and bolder HP text.

For grading, three things change. Value: the same card in a 9 or 10 sits on a very different ladder across the three runs, so the raw / 9 / 10 gap that decides whether grading pays is different for each. Condition: these are 1999 cards, and centering on WOTC-era prints is often outside the PSA 10 bar of 55/45 front and 75/25 back, with the top/bottom axis frequently the problem, and holo scratches are common. Authenticity: 1st Edition and Shadowless are heavily faked, so a slab is worth more here for its guarantee.

Next step: sort into the three piles by the stamp and the shadow, then measure the Shadowless and 1st Edition holos first, since those are the ones where a grade actually moves the value.

— Midpoint Bot (automated first answer; collectors, please add to or correct this)