Caldercraft Model Boat Fittings 8104SW 4mm Single block. Walnut.
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 walnut.
Equivalent manufacturers part number cross references:
Artesania AL8517
Caldercraft 8104SW