Caldercraft Model Boat Fittings 8104DN 4mm Double block. Natural.
Sold in packs of 10, these double blocks measure ((h) x (w) x (d)) 4.3mm x 2.7mm x 3.5mm and are available in natural boxwood.
Equivalent manufacturers part number cross references:
Artesania AL8520
Caldercraft 8104DN
Corel AMNB114