Telescoping Vernier? GNG thread gauges-why?

[Cannonmn]

Pls tell me how these are used, and why anyone would need them. Only one pitch is id’d in the photo, other one same dia but 20 tpi.

 

[guest]

It's not obvious?

Leitech U.S. Limited - COMBI Thread- Depth Plug Gage

 

[gbent]

The purpose is to check a threaded hole for acceptance by a go gage and minimium thread depth at the same time. Its a lot easier than counting turns on the go gage, or zeroing a digital caliper on the overall length of a thread gage and then measuring the length when fully threaded into the hole. The NoGo gage on the back checks for oversized pitch diameter.