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The Lock Technologies decoder in the 70s used a Medeco key blank milled down to a thin spine, then holes drilled in it so it is a guide to position very thin wires under each pin stack. The wires were wiggled until they could be fed up through the sidebar groove on each pin until you felt it hit the top pin. Then you could measure how far the wire went up to decode that pin stack. Those rings would frequently snag the wires so they would stop at the ring and not make it all the way to the top of the pin, giving a false reading.

It was because of the fact that the decoder used a Medeco key blank as an integral part of their tool, that allowed Medeco to sue them and force them to stop production of the tool, since at that time that keyway/blank was still patent protected. All except perhaps a half dozen of those Medeco decoders were tracked down and bought back from the customers at the time of the lawsuit. If you can get your hands on one it would be quite a collector's item, within these circles.

Interesting method.
But even though the full keyway is protected, making a tool which only fits half of the keyway leaving the other half of the keyway empty wouldn't be illegal would it? It is no longer a copy of the key.

In the early 1970's Medeco was quick to make friends in the right government and military circles securing many contracts for high security places such as the White House, nuclear reactor sites, military sites, military padlocks, etc, so when that lawsuit happened, they had a lot of friends in high places to make sure they won. a small shop like the guys who made the decoder tool had no chance even if what they did didn't break any patent or copyright laws.

This is what I heard, so take it for what it's worth, it may not be the whole story...
Squelchtone

My very limited life experience suggest there might be something into it.
Admittedly I don't know anything about medeco's well connected friends but I can see how medeco came handy. They made good (or at least it looked like good) high security locks, which weren't too different from normal locks. And in such sensitive areas I would guess that the company's nationality is crucial.

all this information is a great read but does very little helping me to get two medecos back in service even after trying to get help from medeco im needing 51s cylinder and plug with key the locks are vintage brass bodies and i cant find a cylinder anywhere not to mention plug and key?

Pictures would help for one thing. 51S is not the model number, just the UL listing.
If you don't need to match an existing keyway/system, eBay will probably be your best bet for a reasonably priced Medeco cylinder.