As the title says, I want to convert a skiptooth New Departure Model D hub to 1/2", I have several old Bendix hubs laying around collecting dust, and was wondering if the cogs would swap out.
No--Bendix used a retainer ring to hold the cog on and New Departure was a threaded type. ND did make 1/2" hubs so you can just hunt one of those cogs down to guarantee a fit. It's possible that other thread type cogs will work though since I think the pitch was pretty universal for american bikes at least.
The thread style is usually a bear to remove because it's had 70 years worth of pedaling tightening it down. If you go to take it apart, the lock retainer is reverse threaded, then the cog itself is standard thread.