The oldest manuscripts I mean was the papyrus 66 and papyrus 75, see it in the N7 below.(P66P75)
I also quoted Westcott and Hort's "The New Testament in The Original Greek," with same rendition.
It's not a translation but in Greek.
Here, Updated American Standard Version+, again is of the same context with note N7, and proves that it was recorded as the "original words." Yes, there are variant readings but was not described as original wordings.
(UASV+)John 1:18 No one has seen God at any time; the only begotten GodN7 who is in the bosom of the Father,N8 that one has made him fully known.
N7 John 1:18 (UASV+)The original words were μονογενὴς θεός or ο μονογενης θεος “only-begotten God” or “the only-begotten God” (P66 P75 א B C* L 33 syrhmp 33 copbo) A variant reading is ο μονογενης υιος “the only begotten Son” A C3 (Ws) Θ Ψ f1, Maj syrc).