sphragis

&c. [68] i.e. the sign of the cross in baptism, made with consecrated balm or ointment, and called sphragis in the Apostolical Constitutions, iii.

NPNF1-12. Saint Chrysostom: Homilies on the Epistles of Paul to the Corinthians Editor 1889

And, although the use of 'seal' is not common in speaking of the Logos, Gregory Nazianzen's use of the sphragis metaphor in proximity to eikon language tends to reinforce the conclusion that this is not just an idiosyncracy of Basil's.

The Creed and the Eucharist in the Fourth and Fifth Centuries Rheinische Friedrich-Wilhelms Universität, Bonn 2004

However, the identical collocation can be found in Gregory Nazianzen's 38th Oration (13), complete with the adjective aparallaktos attached to eikon, as in Antioch 341; eikon and sphragis also appear together in his Third Theological Oration (17), with reference to Jn 6.27 as a biblical justification ( 'On him has God the Father set his seal').

The Creed and the Eucharist in the Fourth and Fifth Centuries Rheinische Friedrich-Wilhelms Universität, Bonn 2004

In this list, sphragis appears as a gloss on 'Truth', just prior to 'Image'.

The Creed and the Eucharist in the Fourth and Fifth Centuries Rheinische Friedrich-Wilhelms Universität, Bonn 2004

The unusual use of sphragis in both Basil and Eunomius strongly suggests some common point of reference; it wold be hard to imagine either a casual coincidence here or a deliberate echo one way or the other.

The Creed and the Eucharist in the Fourth and Fifth Centuries Rheinische Friedrich-Wilhelms Universität, Bonn 2004

And he justifies his view by this, that circumcision, which foreshadowed the Christian seal ([Greek: sphragis]), was imposed on the eighth day on those who as yet had no use of reason.

Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 3 "Banks" to "Bassoon" Various

St. Basil at the end of Chapter xxvi. of this work, calls our Lord the charakter kai isotupos sphragis, i.e.,

NPNF2-08. Basil: Letters and Select Works 1895

G, Rutowski R (1991) The function of the sphragis in

PLoS Biology: New Articles Tracey Chapman 2009