The only info I have from my installation manual is it's an Ohlins part # 01691-01 - Nut. Call Ohlins USA and ask them!
Yes, it is bigger for some reason. I have both installation manuals - FG 318 (Ducati 916/748) and the FG 314 (Ducati 999).
Ah, I vaguely remember now - the bike originally had a larger axle nut, but I chewed it up and then put the stock nut back on. There's no obvious reason why the axle nut size matters, and I've had no issues with the stock nut over many years of track riding.