Here’s some useful bibliography, from a new-ish commentary on the Philippics:
I’m sharing it particularly because it supports my strong suspicion that this is indeed also Tremulus on this later type (RRC 425/1; cf. RRC 293/1 illustrated above):
Update 2/15/2016:
Myles McDonnell makes the point very clearly in a footnote (p. 157):
[…] of the type, but not really about identifying the rider. I don’t think it’s Tremulus’ equestrian statue… but the Marcii Philippi’s interest in equestrian themes is […]