Tube designation letters never used

[wa2ise]

While looking thru my copy of "GE Essential Characteristics" I noticed some letters and letter combinations that were avoided. For #xy# x and y were never the same letter. No 6BB6 for example. Other letters were never used at all, like I and O for obvious reasons, but P was also never used except in the early days. 1P5, 6P5 and 6P7 was it. No 6BP5 for example. P and R look similar, but so did E and F and both of those were used. Looks like the tube era ended right at 6MV8. They never made it to 6QA5 or such. Given the pattern, they probably would bot have issued 6PA6...