Do the math: 9.5 mm = 0.374" dia., 1 mm lead = 0.03937" 
0.3937 divided into one inch = 25.4 threads per inch - much closer to 24 tpi than 28 tpi.

I might add the container was also marked as being the proper MM size but that was not what was in side it was the 3/8 X 28UNF along with the note included in the box they where shipped in. If memory serves me right I gave my second tap to Jerry Hartwig? and his container might still be marked in MM's all though it was also a 3/8 X 28 UNF inside mine has since wore off the container it came in or I would have also shared it. 

JLouis

I know you are not trying to be difficult Don and I am only wanting to share what I used on all those I made for others and those I sold. The rear apretures by Gehmaan and others all screwed in very nicely. If you might feel I could be of some help with your project send me an e-mail to the below e-mail address. I still have the information for both a 28 and 30 inch tube sight in regards to the baffle placement and the various hole sizes for each of the four or five baffles I used depending on the tube length one wanted to use. The only real critical part is to insure the tube is completely black inside minus any and all stray light and how one goes about achieving that can very easily vary from one person to the next and that the tube is straight so it will center nicely in the rear mount. When done it should be the same as looking through a scope with all that you are seeing is the front apreture and nothing in between and the same as only seeing the crosshairs when looking through a scope. The way I made mine was pretty labor intensive / allot of time involved all though not at all that difficult to do. Straightining the tube was probably the more difficult part in all honesty if it required having to be done. 

johnnymlouis@gmail.com

JLouis

I might add both taps I purchased were clearly marked 3/8 X 28UNF I just double checked and had I not gifted one I would have gladly gifted it to you. Hopefully Jerry H might also chime in if they were possibly miss marked I would have not known so. If memory serves me right I got the information from Gehmaan at the time for the right MM tap size and you might also want to do the same to be safe I just simply e-mailed that question and then went from there.

JLouis

John, the tap I got from you is marked 3/8-28, on both the tap and container. I used it on a tube sight for Rich$ and it worked, but was tight.

Just going by memory (a bad thing) I once upon a time bought a Gehmann rear aperture from O.K. Weber. He loaned me a tap that he said was as close as you could get because the German threads were unique and not available as a standard tap. Pretty sure he said it was an English Cycle Thread at 3/8-26.

I'm not sure, only saying what I think I remember.

JerryH

Don the way I set my own tube sight up all though I don't recommend doing so for others I can actually line up and center up all five holes in my baffles with the front apreture yet the interior is still void of all and any stray light and something I thought would be an added benefit for myself and it has actually worked out quite well. But I had also incurred some permanent vision loss in my right shooting eye from my stroke and I was trying to make up for it as well. I'am also curently thinking about selling it along with the real nice crisp clicking Unertl mounts but minus the rear apreture due to my eye sight now haven gotten even worse. 

JLouis

Bob send me an e-mail so I can respond to your request.

johnnymlouis@gmail.com