The context that confirms that Potamius' was also giving figurative interpretations of 1 John 5;8(Clause-C)
Potamius of Lisbon
Latin text of the "Epistula de Substantia Patris et Filii et Spiritus Sancti" edited by A. C. Vega in Opuscula Omnia Potamii Episcopi Olisiponensis, Escurial 1934 (pp. 37-54).
Marco Conti, 1998, Page 151.
"Epistula de Substantia Patris et Filii et Spiritus Sancti."
“Igitur, quia superiori tractatu Trinitatis excussimus lumen, corusca sideris claritas rutilavit, Patrem et Filium et Spiritum sanctum in suo sibi themate voluminum omnium interpretabile dogma collegit, superest ut indivisae Trinitatis imperium augustam refulgentis nominis substantiam praedicemus. Age ergo, adiuvante deo, cuius hoc nomen est; de conexu quid sit substantia, dilectissimi, propalemus, antistites. Prius est igitur ut ipsam substantiam legis de auctoritate doceam; ut consequenter et vim substantiae, per quam lex aeterna floruit, de figuris subiacentibus demonstremus. Ait propheta: “Non audierunt vocem substantiae, a volatilibus caeli et usque ad pecora expaverunt et vociferabantur, et dabo Iherusalem in transmigrationem.” [Hier. 9:10]
. Ecce vocalis substantiae locus de Trinitate dicitur; quid ergo facient qui substantiae verba tulerunt?”
“Letter concerning the Substance of the Father and of the Son and of the Holy Spirit.”
“Therefore, because from our previous examination in the treatise [Or: “tractate”]
on the Trinity we were able to elicit forth such light, (oh you flashing star [Or: "oh you flashing constellation"]
), which illuminated clearly all doctrine that is capable of explanation
which we gathered together into all the books [Or: “the volumes”]
by their own theme, the Father, and the Son, and the Holy Spirit. The only thing left over is that I may attempt to make known the substance in a way which is worthy of the splendid name of the supreme authority [Lit., "of the supreme Empire"]
of the undivided Trinity. So let us get on with it then, by the help of God, whose name this is, that we may make known what connection might exist with respects to the substance - oh most beloved bishops. The first thing then, that we might teach is the substance itself by the authority of the law, (through which the eternal law has flourished), and as a logical consequence of this, that we may try to prove to you the power of the substance from the figurative meanings that lay hidden beneath [Or: “from the figures that lye hidden underneath” Or: “from the underlying figurative meanings”].
Yes as the prophet says: “They have not heard of the word [Or: “the sound of”]
'substance' from the fleeing birds of heaven, and the cattle which have become terrified, and they are crying out all the time with a loud voice, and I will give Jerusalem over into captivity.” [Jeremiah 9:10 VL [from LXX 9:9(C)-10(A)]]
Behold! It is capable of speaking of [Possibly: “capable of prophesying about”]
the “substance” in this place concerning the Trinity; but what will they do, then, those who have removed the words about the substance..."