How about MLU (mirror lock up). This was a feature that if it were introduced in the first model, was usually omitted in the following models. Sure, MLU was a feature they reserved for their top models but they even omitted this in their top models as in the examples listed below:
- Olympus OM1 had it then omitted in the OM2
- Canon F-1 had it then omitted in the New F-1
- Minolta SRT-101 had it carried over to the SRT-102 but was omitted in the later production versions
I suppose you can say the Pentax KX had it but was omitted in the MX but they are two different series.
Were there other examples?