Mr_Bill, I suspect you are going to have to replace your existing worm wheel and missing worm with a new matching set of worm and worm wheel. In the US, Boston Gear is one of the very few suppliers of standard catalog worms and worm wheels. (Browning does worm reducers, but not open gearing.) Their catalog makes it clear they only stock right-handed worms and worm wheels. The worm in your photo is left-handed.
Getting a new gear set will require some machining on both worm and worm wheel to fit your dividing head. It should be pretty basic lathe work. Despite the inconvenience and delay, I assure you that buying a new worm/wheel set is going to be less expensive than getting a custom worm made, or the delay in trying to locate a spare (that might not even exist!) for an accessory that was made 50 years ago in very limited numbers.
You know, or should know, that your worm wheel has 40 teeth. That's where the 40:1 ratio between index sector turns and spindle turns comes from. The next thing is to determine the centerline distance between the index axis and the worm wheel axis. You need a worm/wheel pair that matches that centerline distance accurately. Any such pair with a 40 tooth wheel will work. But the centerline distance has to be very, very close (unless you like chewing up expensive gear sets). Better hope that Van Norman used standard catalog gear choices, rather than a custom gear set. One of the complications here that a worm/wheel pair with a given ratio and centerline distance
does not have a given worm or wheel diameter. The gear designer can use a fat worm or a skinny worm, with compensating changes to the wheel diameter. If the designer sticks with a standard pitch, that starts narrowing things down quickly, but even with a given pitch, the designer can still make choices about tooth addendum and dedendum that will affect diameters.
Have a look at the
Boston gear catalog, starting at page 85. On page 98, you will see a table of available centerline distances. Limit your attention to the entries with a 40 ratio. See if any of the matches correspond to the centerline distance on your dividing head. If you don't find a match, resign yourself to a very expensive repair, or a probably futile search.
If you do find a match, you might get incredibly lucky and find that you only need the worm (LH, of course, which would be a special order) instead of the worm/wheel pair. It's not just the pitch diameter that must match the missing worm on your dividing head. It's also worm thread pitch (which is grossly obvious) and the pressure angle (which is not nearly as obvious from a visual inspection) and maybe a few other subtle features. I don't know if that's plausible (due to ancient standardization) or astronomically unlikely. Maybe Zahnrad Kopf or someone who does gears all day long will know. If I were in your shoes, I would consider myself incredibly lucky to find a new worm/wheel set with the right centerline distance, and just buy it without pushing my luck any further.