Thanks to a kind correspondent here, I have become aware that the letter of Arius to Eusebius of Nicomedia is preserved in two Latin versions. These are given in Hans-Georg Opitz, Urkunden zur Geschichte des arianischen Streites (Documents on the history of the Arian dispute), in Athanasius Werke, III, pt. 1, 1934. He gives an edition of the letter as “Urk. 1” (Doc. 1). The text can be found at Archive.org here.
On the first page he lists the two Latin versions, rather gnomically as “Cand. Migne L. 8, 1035.” and “Col. Rev. Ben. 26, 93”. His needless brevity has cost me an hour of my life, and doubtless others the same, so I thought it worth indicating where these might be found.
“Cand.” is Candidus Arianus, whose letter to Marius Victorinus quotes the letter of Arius. “Migne L” means the Patrologia Latina, vol. 8, col. 1035, and it may be found online here.
“Col.” is manuscript 54 of the cathedral of Cologne, of the end of the 8th century. It was published by D. de Bruyne, “Une ancienne version latine inédite d’une lettre d’Arius“, in: Revue Bénédictine 26 (1909) 93-95. This isn’t online, so I attach it at the end.
The Candidus text in the PL is of course a pre-critical text. But there is no question as to what it says, on col. 1037: “ante tempora et aeones plenus deus, unigenitus, et immutabilis” – “before ages and ages fully God, only-begotten and immutable”.
The Cologne ms reads:
Here’s the edition of both in Optiz:
Both say plainly “plenus deus”, fully God.
Since the De Bruyne article is out of copyright but inaccessible, I’ve uploaded it here:
My thanks to the correspondent who sent it to me!
Updated July 1 2019 to add the De Bruyne article material.