One special application rule was the Pickett C-19 Collins Radio Microwave slide rule.   They also made a C-18.   I have a number of C-18s in my collection also.   I found my first Pickett C-19 slide rule in 1993.   I thought it was a very interesting slide rule since I worked for Collins Radio/Rockwell International in Cedar Rapids Iowa in the mid to late 1970s.   It was the first company I worked for out of college.   I did not work in the Microwave Division but I was familiar with some of the scales.   This slide rule reminded me of my college days study for an Electrical Engineering degree.  

Then in 1996 I met a retired Collins Rockwell engineer who had worked in the microwave and transmission area.   As most collectors do, we talked about past experiences and since we both worked for Collins Rockwell we had a common thread of interest.   During our conversation he mentioned that Pickett made some mistakes on the C-19 slide rule.   He did not know what mistakes were made or which scales were in error on the slide rule.   He had only heard of the mistakes.   Pickett reissued the C-19 slide rule without indicating the mistake or using a different part number.   He thought there were two reissues.   This would indicate that there are three (3) different versions of the C19 out there.   If you take the two different colors into consideration, that would make a total of six (6) different versions of the C-19.   However, I have only seen a white C-19.   So the number may only be 3.  

Two years later this gentleman offered to sell the actual Pickett C-19-T slide rule that he used while he worked at Collins Rockwell.  

I have happen to come across three C-19 Pickett slide rules since I started collecting slide rules and I believe I have found one of the mistakes.  

I scanned two of the slide rules and the same section of each of them is shown in the figure below.  

Please observe the Ndbmp0 scale on each slide rule.   Notice that all the values in the scan of the Ndbmp0 scale on the upper unit are negative.   While the values for only 90 and 85 on the lower unit are positive.   The remaining values of the Ndbmp0 scale on both slide rules were negative.   Although I am not familiar with the Ndbmp0 scale, I do know that any db scale will not change polarity within this range.   Therefore, the Ndbmp0 scale of the C-19 slide rule depicted in the lower scan was in error.