Czy tytuł IMPERATORA jest znaczący?

[Heliodromus]

Have you ever seen this variety?

No, I've never seen it. You could try asking the BM if it really exists.

I agree it's probably an attribution/recording error. Maybe a really worn eastern Iovi, or just a clerical error? If it was real, we might expect to see it on GENIO POPVLI ROMANI and SPQR OPTIMO PRINCIPI too, but the "IMP C" and long legend really don't fit for Licinius at Rome.

I don't think the mints were concerned about making the legends for all emperors the same length, just long enough to fill the space. The lack of consistency across mints, as well as variations within a single issue, seems to support the theory that this was not a matter of policy but rather practicality. At different mints at different times there do of course seem to have been preferred legends, but there also seems to have been some degree of flexibility. I wonder if the engravers were explicitly told "it's OK to drop the IMP or PF, or use P vs PF if you run out of space", or perhaps it was just understood what was an acceptable variation or not.

Ben

[Lech Stępniewski]

or perhaps it was just understood what was an acceptable variation or not.

Probably it was a common knowledge what is acceptable and what kind of convention is now in use. Maybe sometimes on some occasions (quinquennalia etc.) the orders were more specific: "give only full long legend for our honorable august Licinius..."

[DzikiZdeb]

The leading IMP seems to be a formality, automatically going hand-in-hand with the title of AVG(ustus), but not in of itself implying any different level of seniority or accomplishment. I'm not sure whether this "automatic" (upon becoming augustus) leading IMP was part of the "imperator count" (e.g. IMP XI in Severus's title above), but even if it was the nature of it appears to be different. The leading IMP is a formality that can effectively be assumed for an augustus, whereas the "imperator count" is at least nominally "won".

Some emperors use other title as their favorite and intentionally ommits "IMP", i.e. Hadrian (HADRIANVS AVGUSTVS on later coins) or Antoninus Pius (his cognomen first, next Augustus, Pius then PP).
Some use first IMP after next acclamations related to military victory (Domitian), some ommit it (Marcus Aurelius)

I wrote about it here: https://www.forumancientcoins.com/board/index.php?topic=124267.msg747942#msg747942

Times have changed and the analogy must come from the fourth century.