The Controller slide rule family

    First, while the three smaller slide rules are produced to the same high quality, the giant 30 cm device -- which would naturally be the most expensive -- has been constructed in a markedly shoddier manner than its smaller siblings:

• It has a cursor made of a flimsy orange-colored celluloid (which had broken and needed me to repair it), where the others’ cursors are made from a thick transparent plastic.
   
• Its central pivot is a simple rivet, where the others use elegant nut and bolt hardware.
   
• The scales on its outer disc are very slightly misaligned. The numbers at the edge of the disc should all have a small, identical spacing from the edge of the metal; instead, they are too close to the edge -- meaning the disc should have been a millimeter or two larger -- and not all to the same extent: the “6”, “8” and “9” touch the edge, while the “3” is spaced 1.5 mm away from it, with the other digits in between. A small imperfection, but completely absent in the three smaller devices.

What is going on here? A likely hypothesis is that the large unit is a prototype, not a final product. This would also explain why it has no model number. Admittedly neither does the 7.5 cm unit, but that is because it was the first model made, and is evidently from an early batch, so it had no need of a model designation to differentiate it; later units this size are marked Mod. 75, in analogy to my Mod. 110 and Mod. 200. The large device also lacks the Controller name on the front, an unlikely omission in a production unit.
    In support of this theory comes the following information from the German eBay seller I got it from: he knows a man who used to work at the Controller calculator company, and this guy informed him that the company had shut down before it could sell the 30cm size in quantity -- in fact, he had never seen any advertisement published for this product.

    The second mystery has to do with the overall design of these devices. There are a number of puzzling attributes they have:

• They contain very few scales -- four, to be precise. This might be excused in the smallest size, but not in the larger ones. Consider the comparison of the 11 cm model to the Gilson Midget slide rule in the following photo: the Gilson, also made of metal in the same diameter, has 11 scales on the side shown, and an additional 11 on the back.

• They all have the same scale pattern, which is merely enlarged from one to the next. This in contrast to using the larger sizes to add precision, i.e. more tick marks subdividing the scales. The length of the main scale on the 30 cm model is four times that of the 7.5 cm’s; yet the space from 2.0 to 2.1 is divided in 5 in both. Given the precision demonstrated in printing these 5 ticks marks on the small model, they could have put 20 tick marks on the large one, allowing one to read the scale with much better precision.

• The cursors on the three production models are made of sturdy Plexiglas or a similar material -- and this is bent (by design) in a way that keeps the hairline away from the scales. This is a big no-no, as it introduces parallax -- if your eye is not placed exactly above the hairline you will not be able to read precisely the number under it.
   
• The moveable central disc is not flush with the outer ring, again reducing the precision attainable when reading the main scales.

Hence the mystery: these are very well-constructed devices, but their design makes them inferior as precise engineering tools. So again: what is going on?
    The answer, it seems, is that these are not meant to be precise engineering tools at all. In fact, they are only able to do multiplication, division, and squares and cubes (in two of them, the ones with an R suffix to the model number, the cubes scale has been replaced with a red scale for reciprocals). This is definitely insufficient for serious engineering work, where at least the trigonometric functions are a must.
    This suspicion is strengthened when you read the detailed instruction manual. I have the German version, but master collector Bob Wolfson from the US was kind enough to share a scan of the English version in his possession. The examples in the manual are all about commercial calculations like margins, compound interest and expenses; no rocket science there. In fact, the manual starts with more than a page explaining how to read “the graduation” -- how to read numbers on a scale. No engineer would ever require that.
    The manual also sheds light on the most visually striking aspect of the Controller: the blue and yellow background color of sections of the main scales. This turns out to be a color code denoting the scale resolution: in the white part of the scale (between 1 and 2), each smallest division represents 0.01; in the yellow part (between 2 and 4), each division represents 0.02; and in the blue part (4 to 10), it represents 0.05. This decrease in resolution results from the nature of a logarithmic scale, and is seen on all slide rules; any user with a technical background would have taken it in their stride, without any need for color coding. But the Controller is not meant for them.
    To conclude this matter, here is a little ditty from the German manual, with the best translation I could generate:

Get it? They are actually bragging that their product frees their customers from the need for having a “Köpfchen” -- which the dictionary translates as (informal) brain; wit; intellect, and which I might more charitably interpret as “a head for numbers”. This is a slide rule for the numerically challenged; and evidently there were quite a number of those willing to buy it, because the company must’ve been doing a brisk business for some time, seeing how they’ve introduced all these beautiful models one after the other.
    I have very little information about the company, Controller Calculator KG, itself -- only that it was owned by one Erwin v. Stojkovits, and was in operation in the 1950s and 1960s at various addresses in Munich. The German manual I have is dated 1970.
    And a last, unsolved mystery: how would a merchant, used to the total precision of an adding machine, feel comfortable using a slide rule, which by its very nature gives approximate results? I have no idea...

More info:
    Here is a scan of the English manual, courtesy of Bob Wolfson.