What’s the difference between a Zenith Cal.126/6 and a Zenith Cal.40?

[M4tt]

I came across a battered but lovely Zenith on eBay the other day and bought it without knowing much about it. Comparing the movement to other vintage Zeniths, I thought I had an immediate hit, the cal.40.
However, the watch I bought had indirect centre seconds. Initially I assumed that the relationship would the same as the Omega 26x and 28x. However, I rapidly discovered that there is no centre seconds Cal.40.

Starting again, I realised that the movement is in fact a 126/6/5 and is part of the 126 series. My problem is that I really can’t see much of a difference between the two movements. In fact the only obvious difference is that the Cal.40 appears to have shock protection on the escapement as well as the balance, while the Cal.126/6 only has it on the balance. However, my 126/6/5 does have shock protection on the escapement.

The dimensions, beat, and look of the two appear identical and they appear to share major components as far as I can see. However, the Cal.40 appears to have a legendary status while the 126/6 doesn’t.

Perhaps more surprisingly, the internet seems not to have anything useful to say. So, I thought I’d ask. Are there any Zenith experts out there who can explain my error on confirm my suspicion?

Cheers.