I suspect RIC VI Heraclea 13, so a Diocletian follis AE3.
Yes, RIC 13 is right, but this denomination is called a post-reform radiate, not a nummus (the name "follis" isn't used anymore, since it means something else).
When Diocletian reformed the coinage in 294 AD, and introduced the nummus (worth 25 denarii), he also briefly issued these post-reform radiates (worth 4 denarii), and post-reform laureates (worth 2 denarii). The post-reform radiates were inspired by the pre-reform radiates (antoninianus denomination - 5% silver), but were pure base metal and therefore worth a lot less.