There are still a ton of differences between the two designations. Even the base plates between the two have big differences. The Chevy baseplates have a EGR vacuum port in the front passenger corner which Olds baseplates lack. Also the Chevy baseplate has newer tab style idle air bypass ports at the bottom of the primaries, under the throttle plates. While Olds baseplates lack idle air bypass ports under the throttle plates. Through the Olds do seem to have round ports just above the throttle plates, not sure if those are also idle air bypass ports or timed ports of some kind? Main point is that one has to really know what he or she is doing to use parts from different Qjet applications. I don'-t think the OP is knowledgeable enough to kitbash Qjets together from different models.
Qjets are not carbs to swap different parts around willy nilly. The external differences are only the tip of the iceberg. Another big difference betwern the two is that Olds carb use restrictors pressed into the air bleeds in the airhorn while Chevy airhorns lack air bleed restrictors. Moreover, the two have different fuel mixture dwell setting procedures. Olds you just adjust the IAB valve with the Thexton gage while Chevy requires final IAB valve setting with engine running and the use of an analog dwell meter. This suggests that Chevy uses a leaner idle mixture than Olds.
It boils down to how each GM division did things their own way. The CCC setup is very different betwern Buick 231s, Chevy 305s, and Olds 307s. You can't just cobble the different parts together like with each division's unique body and frame bracing setups.
This all I agree with, EXCEPT what gm meant by their E4ME vs E4MC designation.
The last letter exclusively meant what choke it had. The fact the other differences existed had more to do with specialization to the engine, but we're NEVER the basis for using a 4th letter e vs c.
The nomenclature from Rochester was as follows:
4M is the universal Q-Jet model number.
Now, theres possible first letters... if nothing its an early 'original' style qjet, pre-emissions. Then you get the 'm' carbs, m4me/m4mc where the first m meant 'modified' in an era when most carbs running around werent changed for emissions yet so they wanted to differientiate and added a letter out front. Years later still you got the e4me/e4mc lines because, you guessed it, electronic control came around but manual carbs, mostly in the truck lines, were still non-computer.
The last letter is was and always will EXCLUSIVELY refer to the choke, regardless of any other differences present.
You could, in mid 70s being last i recall being in use, have a 'v' for divorced choke, a 'c' for the integrated/automatic choke on thebside of carb and run iff intake exhaust passage, usually via tubing, and the 'e' for a full on electric choke.
And, when it comes to lettering which is all i keep getting at, that's it. Buying an 'e4me' doesn't guarantee it is a 'Chevy style qjet' at all, and, buying a 'Chevy style qjet' doesn't mean it's an 'e4me'
Also, for what it's worth, buying an e4mc doesn't mean it is an Oldsmobile carb either. Here's just one top of the head example. The e4mc was used on the Buick 350. Living out its last years in the riviera and big cars. Certainly not an Olds engine, and certainly not chock full of Olds uniquely tailored specs.