What is the difference between RIC VI Heraclea 18b and 20b?

This one, which I'm trying to attribute (also from the multiple lot) could be RIC VI Heraclea 18b or RIC VI Heraclea 20b.
https://numismatics.org/ocre/id/ric.6.her.18b
https://numismatics.org/ocre/id/ric.6.her.20b
I have been trying to find the difference between them and can't see why they have two different numbers.

This is the Follis:
Æ1
Thrace, Heraclea, 296 - 297 (or 297 - 298) AD, Officina ∈ = 5th officina, struck under the authority of Diocletian

Ob.: GAL VAL MAXIMIANVS NOB CAES Laureate bust of Galerius to r. Border of dots.
Rev.: GENIO POPV-L-I ROMANI Genio standing to l., nude, wearing chlamys over his l. shoulder and modius on his head, holding patera in his r. hand, from which liquid is flowing, and a cornucopia in his l. hand. Mint mark -/-//HT∈. Border of dots.





I am very grateful if someone can tell me what the difference is.

 

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Heraclea 18b has a smaller head and Heraclea 20b has a larger head. This can be fairly arbitrary and RIC VI even mentions in the footnotes that this is "to some extent a matter of opinion." There are several times when RIC makes these odd choices, so I catalogue these like this-- RIC VI Heraclea 18b/20b

 

Thank you so much @Victor_Clark for your explanation. You really made my day
Will catalogue it as you do.