It's 1/4" npt. Most drain plugs are 1/4" npt.
The newer drain plugs are designed with an O-ring.
The O-ring plug is preferable if the pump hole has a flat spot for the O-ring to mate with.
A 1/4" npt plug will work in any case.
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1991 and earlier, it's the regular plug (SPX1600V).
1992 and later, it's the O-ring plug (SPX1700FG).