"Surely the B&G chips are a mixture of 39 and 40?"

I have never seen a 39MM B&G.

3EB are 39MM Chipcos.

ALL B&G chips have a small B&G on them.
No B&G on them, then they are Chipcos.

Above applies to chips made for the US.
But
Chipco told me they can not make 40MM.
B&G told me they can not make 39MM in the ceramic style of chip.