...and the 5-hole screwed bezel was not originally its most obvious characteristic.
1955
Instead it was conventional looking watch with excellent shock and water resistance and phenomenal magnetic resistance.
It continued that way through the 1960's and early '70s with relatively few changes:
1969
When Genta produced the Jumbo it had his own quirky design but still kept the Ingenieur's essential qualities. It was a flop: maybe the time, maybe the design and it is a relatively recent rewrite of history which makes Genta's quirks the essence of the watch.
1977
I actually prefer the skinny Ingenieurs of the late 1980's and '90s. In some of them there was evidence of truly serious technology:
1993
...but even the standard models stuck with the original Ingenieur qualities:
1994
My interest dropped off with the Ref 3227.
Although I don't hate it I find it a bit big and clunky to wear. Further, I didn't like the fact that the new movement was so large, IWC were nailing their colours to the large watch fad (my view, YMMV).
2005
Anything more recent than the 3227 is either too big or doesn't have the Ingenieur essential qualities, though I admit some look nice.
I probably obsess too much about the large watch thing but I would be glad to see the reappearance of a 38mm +/- watch with the traditional Ingenieur qualities and a relatively conventional shape. However, the other problem is that IWC prices are getting more and more out of my range, so even if one appeared I don't know if it would be for me.
I've declared victory in my original aim of collecting one of every Ref of Ingenieur by redefining those which don't correspond to the original essence, and those which I don't approve of as not 'real' Ingenieurs. This has saved me a lot of money in the last few years.