I've just bought a 6x4 concrete lintel to be positioned above a window
(was a door) in the inner half of a solid 9" brick wall in my 150 year old
end terrace.

I think the parallel sides should be horizontal, not vertical (large side
down.) Not sure how you make good on the outside

I was told they can be used eihter way up - ie for one brick or two brick
widths - or else they would have to make two different 6 x 4 lintels?

Although it may not matter in this case, bear in mind that the load
capacity of a 6x4 on edge will be more than twice (actually 2.25 times)
as great as with it flat.

--
Charles Lamont