Developed in the mid 40's to set a standard for dial indicators, American Gage Design (AGD) indicators come in four different Group sizes: Group1 being the smallest and Group  4 being the largest.  Group 2 is the most popular and what is primarily availble at Gaging.com.  These indicators have a dial face from 2"-2 3/8".  All AGD indicators have a 3/8" shank and a4-48 thread on the contact point.