This is an internal groove comparator gage, made by Kroeplin (Germany) but manufactured in the Czech Republic. You'll find these sold by Mitutoyo, SPI, Dyer Gage and many other distributors who have had their names stamped on the body. The newest version is a decided improvement over what you've probably seen before. The old models, if you ever opened up the back, actually had a piece of common string as part of the mechanism. Subsequent models changed the string to steel wire. Kroeplin has recently introduced some high-precision models which are wholly gear driven. Additionally, older models had a larger range. You may now have to buy two different Intertest gages to cover the same span.
The feeler arms typically have a carbide ball braised to their ends. This assures consistent readings over the entire range. Unfortunately, the balls will eventually break off resulting in a costly repair. A few models still have flat (chisel) feeler arms but these wear down unevenly and result in inconsistent readings over time.
Since they are made in Europe, the certificates are not NIST traceable but all new models come with a thorough factory certificate of accuracy which is traceable to international length standards. If you need a long form certificate that can be arrange for a fee (and about 2 weeks extra delivery time).
The measuring depth refers to the length of the measuring arms, shown as L in the diagram above. Note that the arms are thicker near the top, which may make some part of L unusable in small holes under some circumstances. The smallest dimension printed on the dial is also the smallest diameter into which the arms will fit. There is an extra .010" leeway but that can vary and shouldn't be counted on.
Dimension A is the maximum depth into which the arm will fit. Dimension B is the minimum width into which the end of the arm will fit. All of these models have the new steel wire construction and carry either the SPI or the Dyer name.
Also known as an internal dial caliper gage, the Intertest will measure recesses and grooves where ordinary dial calipers are useless. Although, the gage is designed to be a comparator, it can be used for taking direct readings as well. You should verify its accuracy as often as possible to make sure the readings you are taking are correct. When used as a comparator set the gage to your master piece and then compare it with your work piece. The gage is most accurate when used in this manner, and some manufacturers (Mitutoyo for example) insist that this is the only way to use the gage.
It is the user's responsibility to verify accuracy before each use of this gage. Be sure to practice taking readings before the Intertest is actually put into service. You don't want to find out that you've been reading it wrongly after it's too late.
The Intertest is available in two range types: the short range and the 1" range. As of 2011, most older models have been replaced by new ones, usually with longer measuring ranges. See the selections below.