Originally Posted by FBinNY

There's a sound structural reason for the locknut to be "capped" (in the OP's words). It may not be as relevant these days, but historically the fork had a keyway milled into it. This weakened the tube and made it vulnerable to splitting if there ther radial loads applied from the inside.

The locknut is capped to reduce the ID to a close running fit on the stem, so as to support it against the side loads as the bar is worked. without that radial collar, the effects of stem flex would be taken up by the fork, and might split it at the keyway. As it is, even with "capped" locknuts, I still run into split forks from time to time.

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IMO you aren't going to fine a commercial nut with 1"x24tpi threads that easily. If you have a aluminum lock nut, you can always ream out the hole to 1". But an easier, sounder solution would be to stack spacers on the fork so the locknut fits properly.