Monobloc and concentric main jets . Interchangeable?

[Ixion]

Refresh my memory. Are they?

I have a box of monobloc ones, but (I think) no concentrics. I need concentrics.

EDIT: Mk I concentrics.

[Motu]

Errr,umm.....I think they are.I haven't even got any Amals left to check.They look different,one is longer I think - but threads and numbering will be the same,it's not like the British to actually change anything just because they made a completetly new product.