Unimat SL/DB grinding arbor. Photo from the Internet
Arbor’s lenght 18 + grinding tool + 3 + 5.2 (bolt head) mm. Thread length on bolt 13,2 mm, M12x1.
Thread on the arbor M12x1, continuous, last 5 mm toward the headstock are bored to ~14 mm.
Holder diameter 15 mm, holder lip 3 mm. Max diameter 28mm.