I just came across this online somewhere and to me, it doesnt sound possible:

The 9.40 was an advanced unit intended for a high spending market. New collectors are surprised to hear that it shares every one of its components with another Longines movement, the 10.40, also released in 1926. The only difference between the two is an extra ligne in the diameter ( the ligne is the unit of movement width measurement traditionally used in the Swiss watch industry) of the latter and otherwise, these units are identical. The running speed of the 9.40 was 18000 half beats per hour, which at the time, long before the fast beat era of the 1960s, was considered a rapid pace.

I suppose it is possible that just the base plate was larger and everything else was the same?

I have a 10.40 with a broken seconds hand pinion and wonder if I should now widen my search?

 

Can't comment on the Longines movements you refer to, but this sort of sharing of parts is very common, particularly in Swiss watches.

 

So it is as wide as that? I thought the whole point about Longines was that they didnt buy the raw ébauche from an ébauche manufacturer, but that all movements were made in house?

I must be using that reference wrongly: All I get is a two parts from the 10.40 movement. But you are right, it shows the balance staff and mainspring and then you can cross-check that those parts interchange between the 10.40 and the 9.40. I wonder if the wheel which has the seconds hand pinion on it interchanges with any other movement.

 

Just because a manufacturer makes their own calibers doesn't mean they won't use interchangeable parts. It's cost effective.