For the early models the top of the case was screwed onto the base with a normal, right-turning thread. For later models there is a left-turning thread. I asked Curt Herzstark why it was changed. Here is his answer:
"When screwing on the top it was possible that the crank would also turn a little bit to the right. This caused the machine to be not ready for use. There were several locks that prevented it. To release these locks one had to turn the crank until it came back to its resting position. This was not only laborious but also annoying. The left-turning thread prevent this source of errors - the crank cannot be turned left anyway." Indeed, a small turn of the crank is enough to block further operation of the machine. The real purpose of the locks engaging in such a manipulation is to prevent operator errors. Several locking mechanisms had already been protected by patents in an early stage, showing their significance. I won't go into details here.
Curt Herzstark told me another nice story about the case. During a visit to the CONTINA AG, Prince Heinrich of Liechtenstein brought a leather case he had been letting made for the CURTA. "Such a delicate machine has to be cuddled snugly, the tin case is much too rigid.", he said. Director Herzstark wanted to see the thing more closely. When handing it over, the machine almost fell to the ground because the leather top cover became loose. This took care of that new idea. During some time the tin cases were replaced by plastic ones. According to Curt Herzstark they didn't meet the requirements, so one returned to the tin cases. That's why nowadays it is the final aim of any calculator collector the possess a CURTA in a tin case with a normal thread !
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