Also a fascinating story behind the inventor Curt Herzstark (born in 1902 in Vienna).

From his very early years he was able to tinker in his father's workshop simply called "Austria" the first (mechanical) calculator plant in Austria-Hungary, back in 1905.

Because of his Jewish heritage the company was taken over by the Nazis in 1938 and he was later put in Buchenwald in 1943. An excerpt [0] taken from an interview [1] in 1987 (1 year before his death):

While I was imprisoned inside Buchenwald I had, after a few days, told the [people] in the work production scheduling department of my ideas. The head of the department, Mr. Munich said, 'See, Herzstark, I understand you've been working on a new thing, a small calculating machine. Do you know, I can give you a tip. We will allow you to make and draw everything. If it is really worth something, then we will give it to the Führer as a present after we win the war. Then, surely, you will be made an Aryan.' For me, that was the first time I thought to myself, my God, if you do this, you can extend your life. And then and there I started to draw the CURTA, the way I had imagined it.

After the war he was nearly ripped off by the Prince of Liechtenstein
(i.e. his financial backers)[0]:

It was not long before Herzstark's financial backers, thinking they had got from him all they needed, conspired to force him out by reducing the value of all of the company's existing stock to zero, including his one-third interest. These were the same people who had earlier elected not to have Herzstark transfer ownership of his patents to the company, so that, should anyone sue, they would be suing Herzstark, not the company, thereby protecting themselves at Herzstark's expense. This ploy now backfired: without the patent rights, they could manufacture nothing. Herzstark was able to negotiate a new agreement, and money continued to flow to him.

One important "innovation"/"simplification" for miniaturizing the calculator to a single complemented step drum was the application of the nines' complement [2]:

The Curta calculator used the method of complements for subtraction, and managed to hide this from the user. Numbers were entered using digit input slides along the side of the device. The number on each slide was added to a result counter by a gearing mechanism which engaged cams on a rotating "echelon drum" (a.k.a. "step drum"). The drum was turned by use of a crank on the top of the instrument. The number of cams encountered by each digit as the crank turned was determined by the value of that digit. For example, if a slide is set to its "6" position, a row of 6 cams would be encountered around the drum corresponding to that position. For subtraction, the drum was shifted slightly before it was turned, which moved a different row of cams into position. This alternate row contained the nines' complement of the digits. Thus, the row of 6 cams that had been in position for addition now had a row with 3 cams. The shifted drum also engaged one extra cam which added 1 to the result (as required for the method of complements). The always present ten's complement "overflow 1" which carried out beyond the most significant digit of the results register was, in effect, discarded.

For playing around here [3] a Curta sim.

[0]https://en.m.wikipedia.org/wiki/Curta#History

[1]https://umedia.lib.umn.edu/item/p16022coll92:50

[2]https://en.m.wikipedia.org/wiki/Method_of_complements#Practi...

[3]https://satadorus.eu/x_ite/yacs_2_0/yacs_2_0.html