Well, they only sell one distributor. So logic says it's not them. The only option is a steel gear for pre 1954. Did you buy the distributor yourself? Has anyone replaced the oil pump?
There online pic shows a gasket/spacer and an Oring.
Here are my guesses:
1. You are NOT in the slot
2. Someone has been playing with the Langdon distributor
3. It's a home made "Langdon" copy
4. It's all together the wrong Dizzy
5. Most of these guesses will be wrong if you yourself actually ordered it from Langdon. Seems like if you did you would have called or emailed them.
6. I see your hobby is "gears" hmmmmm, did you play with this dizzy?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn