... τουτον γαρ ο πατηρ εσφραγισεν ο θεος. [John 6:27 TR, undisputed]
... this he for the father did seal the deity. [Literal]
... for him did the father seal even God. [YLT]
If one removes the italic from Robert Young's translation (the italic being an admission that the word is not in the original) then one has an equivalence :
... for him did the father seal God.
'Him' (the Son) is in the accusative as the one being sealed.
'the God' is nominative and therefore has equivalence to the subject 'the father'.
Instead of adding 'even' in italics, would it be appropriate to add 'as' in italics :
... this he, for, the father did seal as the deity. (Proposed)
Does the text indeed state a grammatical equivalence ?
What other reason is there for the word order to be as it is, that is to say the nominative following the verb when a nominative already precedes the verb ?
Is not the meaning that the Father has impressed his seal upon the Son to demonstrate that he is equal Deity ?
EDIT upon Comment :
In context, Jesus has conveyed himself away from those who would make him king based on the free distribution of food to the populace. But his ministry is far more than that. His ministry is above and beyond all that.
This is the context of Jesus revealing his sealing of the Father.
He is more - much more - than a mere king.