I take it the 0.397" is the clear hole diameter. Sharp V double depth for 24tpi is 0.0722", so the o.d. ought to be 0.4692. For standard thread, the double depth is 0.0541", giving an o.d. of 0.4511". Neither of those sounds like anything rational. I'm guessing the 0.397" dimension is somewhat larger than the actual root diameter. I'm with JCHannum -- try 7/16"-24 and see what kind of fit your get.
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
There are lots of people who mistake their imagination for their memory. - Josh Billings
Law of Logical Argument - Anything is possible if you don't know what you are talking about.