Prototype Mk.1 roof vents - all in a row?

Dog Star

Western Thunderer
Standing on a footbridge I, looking over a service train made up of Mk.1s, I noted that the spacing of the roof vents was new to me. Here is a photo of two TSOs (4862 near and 4923 far) with this interesting pattern of vents:-

TSO 4862 (near) TSO 4923 (far).jpg

I was reminded of the Charge of the Light Brigade - two to the left of me, two to the right of me, three in the front of me. Why are the "pairs" of vents grouped at one end whilst the two "triples" are grouped at the other end? I guess that the placing of the vents might have been in relation to which part of the coach was the smoking saloon.
 

mth

Western Thunderer
I think you might find that the middle row of vents was added when the coaches were refurbished.
 
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