RIC VII's description of helmet types is useless, no place more so than during this Rome 318-319 time period!
In reality there are three types of basic helmet (different construction), and two types of crest (feather, horsehair), giving a total of 3x2 = 6 varieties.
The three types of helmet are:
1) The archaic type with protruding cap-like "eye shade", which RIC refers to as "Athenian" (RIC VII p.315 - RIC 60 footnote)
2) The bowl-shaped ridge helmet as seen on the Trier billion, then reintroduced in 318 on the VLPPs and contemporary types at Rome and Thessalonica
3) The "Praetorian helmet", as seen on the "Praetorians Relief", which RIC (same footnote as above) refers to as having a visor, but is really a fixed decorated browband
Each of these helmet types may have either type of crest. The ridge helmets may or may not have a laurel wreath, independent of crest type.
Of course in RIC VII's bust-type descriptions it doesn't get into these details, other than an occasional "high crested" clue (feather crest) narrowing down what they may be talking about !
Comparing bust type to coins, we can see that some helmet types can *usually* be inferred:
A D6 bust has a ridge helmet with a (typically low-lying) horsehair crest. By definition a D6 bust is also laureate, but sometime on a "D6 var" it isn't.
A D7 bust has a ridge helmet with a tall feather crest (with narrow attachment point). By definition a D7 bust is NOT laureate, but sometimes on a "D7 var" it is.
A D2 bust has a Praetorian helmet
Unfortunately the bust descriptions of these Rome P-R coins leaves a lot to be desired, starting with the fact that they use "D2" to indiscriminately refer to busts with either Athenian or Praetorian helmets. Then, having used D2 to refer to some coins with Althenian helmets, they only illustrate one coin (RIC 143) with this helmet type, but attribute that one as D6 (seems like an error, should be "D2" per their convention).
The helmet on that RIC 178 campgate is a ridge helmet with horsehair-crest (i.e. not a "high" feather crest). That *is* typical for an H4L bust for Constantine here at Rome, but not 100% consistent. For Licinius the helmet type is usually Praetorian, and that is what you'll see on an H4L for him, but I've also seen one H4L with Praetorian helmet for Constantine on a 4-turret no-door P-R campgate.
For illustration, here's an "Athenian" helmet for Constantine I recently acquired. RIC would classify that as a D2 bust, but it's better considered as unlisted vs a "real" (Praetorian) D2 bust.
Here's the Praetorian relief with their fancy-boy decorative browband helmets, with feather crests.
And here's Licinius in his typical-for-him "D2" Praetorian helmet, with a horsehair crest.
