Reviving this thread because I also need 3/8 inch thread speaker spikes for the stands to my Vandersteen 1Cs.
The documentation says they're 1/4 20, but after trying those and finding they were way too small I purchased a few different sizes of carriage bolts at the local hardware store and tried them until I found the right one: 3/8.
These appear to be old Sound Anchor stands, though I can't find any markings on them, and they seem different from any other pair I've seen in pictures. Looks like they weren't stored terribly well and I'll need to sand them down and repaint them.
I've done a lot of searching, and all I can find are 1/4 20 and 6mm. I looked at the sites listed above from a couple years ago, and no luck.
Any advice?