Greetings,
This thread has been most challenging and educational.
Based on SAE Document J476A (1961), (link below) on page 15.22 is a section that deals with non standard threads, such as the powder hopper cover.
So, my latest calculation indicates the thread is a
9/16-27 Dryseal SPL-PTF, OD 0.563.
So far, I have machined and heat treated my "special tap". All that remains is to get it sharpened and test it out. I have screwed it into several powder slide tops and I think I have a keeper.
Another note. There is someone selling powder shutoff devices on eBay. While looking at the photos of his/her parts, the thread did not seem tapered. I wrote and asked...
"Is the threaded portion a tapered or straight thread? My existing powder
hopper has a tapered thread."The answer I received was...
"The threads in the special fitting ,and the threads in the valve are 9/16 -
24...
streaght....it works with my press ,and have sold alot of valves
with no complants...."I suspect that the part starts into the Powder Slide Cover; but, with continued use will damage the original threads. Buyer beware...
http://www.google.com/url?sa=t&rct=j&q=sae.j476a.1961.pdf&source=web&cd=1&ved=0CEMQFjAA&url=https%3A%2F%2Flaw.resource.org%2Fpub%2Fus%2Fcfr%2Fibr%2F005%2Fsae.j476a.1961.pdf&ei=qH4pULjyOePqiwLYr4GQCw&usg=AFQjCNHWOzTtImODBbPwqCiWMW1fQLptTA&cad=rjaCheers,
Dave