These are small test pieces to examine the qualities and machining properties of full color sandstone pen blanks. Each mini-blank test piece is just over one inch in length but the other dimensions are the same as the larger blanks in the "C" series used with 8mm metallo pen kits.
The inside hole diameters as fabricated can accept 7mm brass tubes used for pen making but may need a very slight amount of filing before gluing the tubes into the blanks. If you would like to perform tests using 8mm brass tubes you will need to file out the inside to a slightly larger inside diameter. Either way the best method I've found to do this is to use a round file and carefully rotate the piece as you file to keep the hole centered within the tube.