Dutch army, Johan Herman Fuhler, somewhere between 1945 and 1950
According to the Spare Parts List....Rear Chain Adjuster Bolt...3 1/8" x 5/15" x 26tpi.
It does of course have a small hex. If you need photos or a sketch, then just ask.
Thank You very match for Your help.
If you can, send me photos and drawings of these screws on my email.