I believe you will find your answer here:
 
https://grandbrass.zendesk.com/hc/en-us/articles/201711490-What-is-IPS-and-Why-Do-We-Use-These-Size-Threads-for-Lighting-Parts-
 
Tom Tillson

---------- Original Message ----------
From: Nick England <[email protected]>
To: Greenkeys <[email protected]>
Subject: [GreenKeys] Weird hardware size
Date: Fri, 25 Nov 2022 14:21:27 -0500

Well, Teletype is kind of weird in using 5-40 and 6-40 where most folks would use 6-32. 

And using 1/4-32 instead of 1/4-28. 
But who in the world decided on 1/4-27 for lampshade finials? Where did *that* come from?

--

Nick England K4NYW
www.navy-radio.com