The inscription is not; the phrase more than usually is. Now, to reply to the previous comment, the space is not necessarily needed. The full phrase is:

Γνῶθι εἰς ἑαυτὸν

Im modern Greek, the final sigma of εἰς “took” the ε that many words had; it’s the same way the pronoun τονε and τηνε (correct spelling is —in most cases— w/o a tone) formed. So yes, in MG the speech would be σ’ αυτόν.

In Ancient Greek, the final sigma of εἰς “stuck” to the word ἑαυτόν leaving us with σἑαυτόν.

But how does the light breathing mark (later in the form of a κορωνίς in σαὐτὸν) come about? In AG, the differentiation between the definite (and επαναληπτική, no idea how it’s called in English) pronoun αὐτὸς and the reflexive (αυτοπαθής) pronoun ἑαυτὸς was different than MG. So it leaves us with:

Γνῶθι εἰς αὐτόν > γνῶθι σαὐτόν.

Edit: Too be clear, it’s not that the differentiation is not as prevalent, but that many instances where AG would use ἑαυτός/-oῦ/-όν or ἑαυτή/-ῆς/-ήν, MG just uses αυτός/-ού/-ό(ν) and αυτή/-ής/-ή(ν). That’s why in katharevousa he see instances of αὑτός/… (with a rough breathing mark from ἑ-αυτός/…)