Reed seat size for Northumbrian pipes

Just picked up a set of Northumbrian drones. Two of the bores are just over 3mm and two are 4mm. Is this a standard size for drones? That’s an awfully small hole to get a drone reed in.

Takes small drone reeds.

Guess I’ll have to get busy and finish these tube drone reeds then. Might also try some of the plastic double reeds that I make. I did send some of the latter to a piper with smallpipes. He says they are working fine.

I make cane drone reeds for UP, NSP, SSP, GHB etc.