Starrett 823 ID Mic Thread Size

Does anyone know offhand what the thread size is for the extension tubes on a Starrett 823 tubular ID mic? I've been doing some line boring, and want to make a C shaped anvil to straddle the bar to get accurate measurements. The solid rod ID mic works OK, but I have to go thru the bar at the 1/2" toolholder hole, and depending on the length of the bore, it's a pita to keep moving the setup. Also doesn't have a lot of clearance. I know, I know, I can measure the threads myself, but didn't have the bifocals with me , or a magnifyer. I'm pretty sure I'll have to order a tap and die. Thanks, guys Ron

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