What is it?

[horseman2]

Appears to be a for measuring to the shoulder of a case.
The silver part is tapered and removable.
The inside measurement is 0.379 which would be for the neck.

 

[yomomma]

Bullet comparator

 

[horseman2]

I have a comparator and it is different in that the inside is tapered more like a shoulder measurement device.
The inside measurement of 0.379 would allow cases up to 35 caliber to me measured.

Made a Query of Sinclair and it is a bump gauge.
They list a 20 and a 20A but no 20B.

The 20 signifies the angle of the shoulder and the 243 range of cartridges is in this category.
Waiting for a reply via email.

I put a couple of notifications when available and dropped another on to my order. It can wait.

2021-02-12: Contacted Sinclair's Tech people and the 20B has a larger opening. The 20 is for 243, 260, 7mm08 and 308. The 20A is for 338 Federal and the 338 LM. The 20B would be for the 358 Winchester or in that category, maybe a 338/08.

 

[RickF]

It is a Sinclair tool. You can buy inserts to check either bullet ogives for seating depths (as per the second photo) or as a way to set resizing dies and measure shoulder bump.