Caldercraft Model Boat Fittings 8104SN 4mm Single block. Natural.
Sold in packs of 10, these single blocks measure ((h) x (w) x (d)) 4.3mm x 2.1mm x 3.5mm and are available in natural boxwood.
Equivalent manufacturers part number cross references:
Artesania AL8512
Caldercraft 8104SN
Corel AMNB101