Interapid 312B-15 and 312B-15V (vertical style) already come with 2.75" long contact points. You have the option of installing a 5.5" long point (exactly twice the length).
This point can also be used with a Mitutoyo test indicator with M1.7 threads. There will be no correlation with the graduations, however. You should use it only for indicating and centering purposes.
You will have to make sure that whoever uses the indicator knows about the change in value. This extra long point is not sanctioned by the manufacturer. If you use it, all guarantees of accuracy are void.
Many customers use this long point for finding center and concentricity of barrels, such as rifle and gun barrels. In those cases, the value of the graduations are of little concern. Customers who have used this special point report that the indicator works fine and that the overall accuracy is not compromised.
The shaft of the point is steel and the ball is carbide with .080" diameter. The thread is M1.7 making it suitable for Interapid indicators. It should not be used with other Interapid models. The 312B-15 and 312B-15 have stronger hairsprings which helps overcome the extra weight of the longer points. We do not advise using this point with any other brand of test indicator.