Mike, found what I have. Turns out they are boring head arbors (2), 7/8-20 and 1 1/2-18 instead of holders. Don't know why I thought they were end mill holders. Been put away for a long time. Problem is though, is that they are 200 series, not 300. Sorry to get your hopes up. They're still around though, I've sen them on the bay, while searching for possibly an adapter or some way to use these I have. never did find anything useful.