Morphophonemic rules in phonology

Question

I am wondering how the two phonological terms "morphonemic rules" and "morphophonemic rules" can be distinguished?
A morpheme might have different presentations (i.e. pronunciations) and as far as I know, morphonemic rules are utilized in order to specify different phones (pronunciations / realizations) of that particular phoneme.
Example:

The plural /s/ sounds [z], whenever a noun ends in anything else than <+sibilants> or <-voice> segments.

According to what we were taught at the university, this is considered to be a morphonemic rule, but what are morphophonemic rules?

Answer 1

The answer is that these are just variant pronunciations (and spellings). The overwhelmingly standard term is morphophonemic. The haplological alternative arose in the late 60's and 70's but subsided in frequency. Perhaps some day someone will undertake an in-depth study of these terms in the literature, but no such study has been undertaken, thus you have to rely on intuitions of phonologists who have been reading phonology over a long period of time. It should be noted that either term is archaic and does not correspond to any generally-accepted theoretical concept of modern phonology.

As a poor substitute, you can inspect the Google Ngram of those terms, and you will see that morphophonemic is 100-fold more frequent than morphonemic. The Ngram viewer will also lead you to historical instances of usage, where you can try to get the meaning from context.