I had to get out my old machinest handbook printed in 1941 to find the dimensions. The book was given to me by the man who taught me the trade. I've about wore it out through out the years. If you look at the sizes the #5 is exactly 1/8 of an inch so it was probably fairly common back in the day. Also the #4 and #5 were the only ones to come in 36 TPI. The most common number of threads for both of them are 40 and 44 TPI. That shop I started in had both #5 taps in 36 and 40 TPI. The man I worked for had worked for United Airlines as a machinest during WW2 so those sizes may have been common on airplanes. Mitch