Well, I see rather D7 with a long crest.
I don't believe there is any such thing, although
RIC VII's helmet descriptions are so useless that it's not as clear cut as it should be.
In reality there are three
types of helmet design/construction seen on the coins: archaic/pseudo-corinthian, ridge and attic/praetorian (as on praetorian relief), each of which may have either
type of crest: feather or
horse hair (= 3 x 2 total varieties).
Of course
RIC VII doesn't actually describe helmet
types in general, outside of a few clues in footnotes, but rather gives us
bust types such as D2 (& draped D1), D6 (& draped D5) and D7.
Comparing coins to descriptions, it seems that D6 and D7 both refer to ridge helmets, although only the D7 description gives us a clue ("
bowl shaped"). The main differentiator of D6 vs D7 is that D7 is described as "high-crested", which seems to mean feather crest vs horsehair crest (which lays flatter vs standing straight up, typically parts in the middle - falling to front and back, and may flow down the back of the helmet). Additionally it should be obvious from the engraving whether an attempt was made to depict individual broad feathers or more closely
engraved lines indicating horsehair. RIC's descriptions also say D6 is laureate, but in reality each of D6 and D7 may or may not be laureate, especially on these
Rome 318-319 coins, so it's better to use the helmet/crest
type as differentiator.
The D2
bust description only says "helmet", not giving any clue whatsoever what it is talking about, and seems to be used as a catch-all for non-ridge helmets. In practice any
Constantine helmet pre-318 AD, other than
his 313 AD Trier billion (which gives us a preview of
his later feather-crested ridge helmet), is going to be archaic/pseudo-corinthian, and *almost* everything after 320 AD is going to be attic/praetorian (with upright decorated browband, and swirly decoration).
The
Rome 318-319 period (P-R coinage) is especially confusing since:
a) All three
types of helmet, and two
types of crest, are in use at this time
b) As they admit in the footnotes, RIC fails to distinguish between archaic/pseudo-corinthian and attic/praetorian helmets, referring to both as "D2", which turns out to be a rather fatal mistake
c) Of the two plate pictures RIC gives us related to helmet
types, one is wrong. RIC 143 is listed with a D6 helmet, but illustrated with an archaic/pseudo-corinthian one, which
per their convention should be described as D2.
Anyways, I believe D7 "high-crested" is always a feather crest, normally *very* obvious. D6 is a horsehair crest, which may have a fairly broad range of appearance from low lying to fairly high (but then parted, falling to each
side, possibly flowing down back of helmet).
I'd say the coin you posted is RIC 213 with a D6 (horsehair crest) helmet. For comparison, here's RIC 214 with a D7 "high crested" (feather crest) helmet.