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We have a 21C609 at the museum, which is a CTC4 clone. It is in Sams 352, while the 31C606 is in 391. I guess they sold the 21C for a few months until they could get their own design to the market. Strange that it uses a gas regulator tube when the shunt regulator had been in use in almost all sets for 2-3 years.
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