Here is a simple way to tell whether you have the old or new model Girodtast.
Your old models will have the same black bearings on both side of the body. Your new models will have different bearings on either side.
Old models (pre 2008) contact points are held securely in place with a set screw which is inserted from the back end. Swivel the point to the side and you'll see it. A very small screwdriver is needed to loosen this set screw by a half turn. This allows you to unscrew the contact point.
Old model contact points had M1.6 threads and these will not fit the new GL series (see below).
Many Girod contact points have a strange burr above the thread. You may find that you can't insert the new point without stoning off this burr. When the new point is screwed in, you'll want to tighten the set screw again. Notice that, if the set screw is too far out, you may not be able to swivel the point back into position.
This same set screw is used to shorten or lengthen the contact point so that the indicator calibrates correctly. It's quite a nuisance. You'll have to calibrate the indicator and if the readings are over by one graduation or more, you'll want to lengthen the point. Do this by screwing it out, by 1/2 turn at a time and then tightening the set screw. Calibrate again, and again, and again until you get it right.
Why did the manufacturer torment us with this "feature"? If the contact point is used at an angle you will have to make mathematical adjustments for the cosine error. However, on this kind of indicator you can permanently adjust the length of the point so that readings at a particular angle will always be correct.
New models (since 2008) of the "series GT" have eliminated this setscrew making adjustments and mathematical calculations unnecessary. They now operate on the same principle as the Bestest and Tesatast indicators. New model contact points have M1.4 threads and will not fit the older models.
How can you tell whether it is M1.4 or M1.6? Just take your old point and measure it metrically. M1.4 threads will measure 1.4 mm in diameter.