The Extendadot came about after years of frustration trying to slow down the dots on speed keys.  The design distributes the dot arm weight equally across the entire length of the dot arm and that provides for better quality dots from low to high speed.  When its adjusted for the lowest speed, the average weight slides beyond the end of the dot arm.  So it requires up to 2" of free space behind the key.  A small amount of weight is added or removed from the end to fine tune the speed range desired.   Originally, I only intended to use one design for the Extendadot.  But as it evolved, I have ended up with several different models.  The basic extender allows a small end weight to be placed  vertically or horizontally--  or a combination of both.    The Deluxe Trombone design provides a sliding trombone so that the dot speed can be fine tuned without having to add or remove weights.  The Trombone requires a considerable amount of hand fitting, so the cost will be a little higher.  
The picture shows the Vibroplex 100 year Anniversary key with the Extendadot.  Below the key is pictured the Trombone and the Basic model vertical/horizontal extender shown with both a vertical and horizontal end weight.  All keys have slightly different main springs.  So how much weight you'll need on the end  for your desired speed varies from key to key. If you prefer a higher speed range, you probably wont use an end weight.   What's unique about  the vertical nylon screw is you can slide a small weight on and off easily for quick speed changes all the way down to 10wpm if you desired.   Nylon screws are used in order to keep the weight down for faster high speed dots.  Nylon provides a better grip on the dot arm and takes less force to hold it in place.  The Extendadot is all brass except for the Nylon screws.  Where possible the Nylon is painted with 14K gold enamle to match the brass color more closely.    The large -- 1" high knurled adjuster is much easier to use than a normal small weight screw.     The large size makes it much easier to slide it on the dot arm.      The extender replaces the weights on the dot arm and the entire assembly moves back and forth on the dot arm.  Except on the Lightning style keys with the flat dot arm.  On the Lightning, the extender attaches to the bottom of the existing square weight.  A few square weights didnt have screw threads extended to the bottom and on those, there is no way to attach the extender.  You can see in the pictures an example of how it looks on the square Lightning style weights.  The one pictured is attached to a Lionel J36 Lightning square weight.  The objective was to allow mechanical speed keys to be easier to use on the air.  And the Extendadot serves that need very well.  You can find other interesting info ---- including how to properly adjust a speed key at: http://journals.aol.com/w7is/TELEGRAPHKEY/     Frank W7IS
Contact me via email for further info at:  sales@extendadot.com