I've got (1x) spare of each size end mill holder; brand is import (Shars, I think); unused/new condition.
R8 taper & 7/16x20 threaded shank
These hold endmills more rigidly than a collet chuck since they are locked in place with a set screw.
1/2" -$22
3/4" - $25
Willing to sell outright or...