During that time, Model MC should indicate the Monte Carlo Stock. Model MCA would have a slightly lower comb, those who desired the
traditional stock of previous models were advised to order Model 1952 GK:

Stoeger catalog, 1960

Model GK

Stoeger, 1957

Stoeger, 1962

Magnum

There was also 'model NO' which Kuduae (who knows much more than I) has suggested may have been European custom orders.

One hard, fast, rule regarding MS, however, particularly of that period, was that there were few, if any, hard, fast, rules (my, that was a lot of commas).