This mark was only used on Gloria Exercitvs
, not on the contemporary Constantinopolis
& Vrbs Roma, although the following Aquileia "F" issue included all three active types.
On some of these the cross consists of a serif'ed "T" with a large round dot making the top "upright" (this is the only way Guido Bruck's "Die Spatromische Kupferpragung.." depicts it - see also Num. Chron. 1997), but on others it's clearly a two crossed lines construction. It's not obvious what was the cannonical form, or the intention - a T, some form of cross (commissa/imissa/?) , or possibly a deliberately ambiguous symbol?
Given Constantine's prediliction for T-F control marks, I've wondered if these consequetive marks should be taken that way, but on balance I'm more inclined to assume it was meant as a some type of cross. We do also sometimes see what appears to be a deliberate long cross (vs the more common "+" greek cross) on the emperor's standard of the Fel Temp "2 captives" type.
From what I can recall it was at one of the church councils during Constantine's time that the cross was formally adopted as a Christian symbol - earlier it had been rejected by the apologist Minucius Felix as being a pagan symbol, and it seems quite likely that it's use as a Christian symbol grew out of it's existing usage taking adavantage of the crux immissa symbolism, notwithstanding the crux simplex (stauros) originally attested in the bible (& depicted in some early engravings).