Definitely no need to thank me for the feedback, it was all genuine and provided out of real appreciation for your efforts. You present solid information in a clear, concise, and confident manner, and you put it out there freely for those who are interested. That to me is worthy of praise and gratitude. I first entered a dojo in 1967, back when Judo was the most popular Asian martial art in the USA (probably long before you were born!). In the 60's and 70's, there was still a lot of secrecy and closed doors in the martial arts community. To have had access to the kind of information that is available today was beyond imagination back then. The technology has made the sharing possible, but it still requires skilled individuals such as you to be willing to take the time and effort to put the information out there, so again, my thanks to you.
Please feel free to use any information that I provide to you as you see fit. Keep in mind that some of it would be in the category of "oral tradition" as I probably could not find written documentation to support it, but it's what I recall.
I have attached a photo showing the items that I mentioned in my first email. The item on the left is the first Kubotan that I purchased in the late 1970s. The word "Kubotan" is almost worn off, but still partially visible at the bottom. The item next to it is a Monadnock "Persuader". You can see that it's a little bigger than the Kubotan, but the most notable difference is the raised ridges on the shaft instead of the typical grooves. The ridges do provide some improvement in grip and they can be more aggressive against the skin during a "gasket" locking movement, but they make the overall diameter larger and would seem to be more difficult to produce in materials other than plastic. The third item from the left is a D-Jammer. Because it was designed to also function as a firearm tool, the diameter is thinner to fit a 9mm barrel and the key attachment is canted at 90 degrees from the shaft to allow the thumb to cover the end when pressing down. The downsides for me were that the slimmer shaft doesn't feel as secure in the hand and the angled key attachment causes the keys to flail a little erratically. My favorite was the style on the far right, closer to the original design but made of aluminum. As it's condition shows, it saw a far amount of time traveling with me.
Once the TSA formally classified the Kubotan and it's variants as prohibited items, I gravitated more toward pens and mini-Mag flashlights as preferred improvised self-defense tools, and these have not raised any concerns during security screenings when placed in a carry-on bag or computer case. No matter what country I was in for work or pleasure, I never had any concern about the local laws and legality of carrying one or two pens on my person. Since not all shirts have pockets, I always carry a pen clipped in my front pants pocket where it's easily accessible and I always know where it is. If the shirt has a pocket, then it's a good spot for a second pen but there's always one in the pants pocket closer to where my hand is likely to start out. As you noted, basic aluminum pens are definitely preferable but there are numerous inexpensive plastic pens that are more than adequate when a defensive response is unavoidable. Even the ubiquitous Bic pen can be a very useful tool when there are no other options.