A particularity of the Speedmaster sub-reference 105.012-66 is that there were two case manufacturers: Huguenin Frères (HF) and La Centrale Boites (CB).
Cases made by La Centrale Boites are slightly different, and Speedmaster models equipped with such cases are called 105.012-66 CB.
In addition to the conventional shape with twisted lugs, there is a special Speedmaster caseband in which the tops of the lugs have a ledge along part of their length followed by a small step. We have only seen this type of caseband on the sub-references 105.012-66 made by La Centrale Boites (hallmarked CB on the caseback).
It is likely that this manufacturer was commissioned ad-hoc in 1966 to make up a probable shortfall in the production of Huguenin Frères.
You can see on this comparator the difference between
the standard twisted lugs (left) and the twisted flat lugs of the CB models (right).
Please note that some of the 105.012-66 cases of La Centrale Boites have had this particularity removed. It is likely that the ledges were considered unattractive so that they were filed away in one or several polishing operations.