This is a 1952 Traveler w/ locking fork (swapped bars, updated SA shifter and pedals). There is just a special sized caliper bolt that screws into that threaded hole. I don't have a locking fork accessible right now, so can't be of more help on sizing or specs. Also below is my 1952 World Varsity with locking fork (I have this in off site storage, so don't have it in hand right now).
The high flange rear hub would not ever be correct on a Traveler or World at the time, but could be correct if that were a New World. They were really pushing the three speed setup on those bikes at the time. The rim and hub are not correct (if a Traveler). To
@SirMike1983's point, the New World and World Traveler were basically facsimiles of one another at that time, moving from the 40s to 50s and I have said it before but the New World essentially turned into the World Traveler. Trying to figure out if that is a late New World frame or an early World Traveler frame is difficult from pics. Also a lot of the hardware/components just rolled over from the New World to Traveler, so again hard to tell. I was going by the decal and other things. If you see any other decal remnants or pin stripes that could help.
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