Ἐφ’ ἕνα ἕκαστον αὐτῶν: why is it ἐφ' ? What is ἐφ' ?

In Acts 2:3 we see the final phrase

ἐφ’ ἕνα ἕκαστον αὐτῶν . . . on each on of them

referring to the 'tongues of flame'.

The preposition ἐπί is a very common word in the New Testament, used about 900 times.  The basic meaning is something like 'on' or 'upon', but lexicons (see, for example, this page in Thayer's) have numerous variations on this theme.

Our question refers to the spelling.  Ἐπί is found in three versions, depending on what letter follows:

1)  ἐπί (ἐπὶ) before a consonant

ὅτι τὸν ἥλιον αὐτοῦ ἀνατέλλει ἐπὶ πονηροὺς καὶ ἀγαθοὺς καὶ βρέχει ἐπὶ δικαίους καὶ ἀδίκους

for (he makes) his sun rise on the evil and the good, and it rains on the just and unjust   (Matthew 5:45)

2)  ἐπ' -  simple elision before a vowel with a smooth breathing 

ἰδὼν τὴν πόλιν ἔκλαυσεν ἐπ’ αὐτήν

having seen the city, he wept over it  (Luke 19:41)

3)  ἐφ' - elision before a vowel with a rough breathing.  The 'π' now changes to 'φ' for reasons of euphony.  Thus in Acts 2:3:

ἐφ’ ἕνα . . . upon one

Also, for example:

ἠκολούθει αὐτῷ ἐφ’ ἵπποις λευκοῖς

(they) followed him on white horses (Rev 19:14)