Odd calipers. I know what they are but I don't know what they are

[Sudsy]

I just finished rehabbing these odd little calipers -
About 4 1/2" long

No idea what were for or how to read em - anyone here have any ideas ?

Made by Grove Mfg. Co. of Dayton, Ohio
All I could find anywhere was this one line:

The 1922 edition of Engineering Directory lists Grove Mfg. Co. of Dayton as the maker of "Grove Indicating" brand calipers.

[Northwoods]

Curioser and curioser!

[turnnut]

 if you set t on zero, what is the ID measurement ?

 same on the 4 on both ends.

[mvwcnews]

( https://books.google.com/books/content?id=NohNAAAAYAAJ&pg=PA82&img=1&zoom=3&hl=en&sig=ACfU3U3GUXwpIZgY82d2d9hsyC02pjE8iQ&ci=10%2C712%2C469%2C779&edge=0 )  Sept. 1918 MACHINERY write-up for the tool, with an illustration of how it could be used.
P.S. Found the patent #1,365,779 which will soon be in DATAMP -- can I use your photo of the tool for that entry?

[Spartan-C]

Poor mans micrometer.

[Sudsy]

can I use your photo of the tool for that entry?

Of course

And here's how I read that ad (I'm far from an engineer so i could be wrong)
OK, I just read it over again, here's what i got from the ad - It's a tool used to accurately measure an inside measurement. It's used in conjunction with a micrometer to match a pin to a hole.

[Sudsy]

http://www.freepatentsonline.com/1365779.pdf