I don't know the answer to your question, but I have a hypothesis you may want to consider.
Maybe there was no noun in "впервые", maybe it was a combination of the preposition "въ" and the word "пьрво"?
I've looked for examples in Sreznevsky's dictionary (compiled in the end of XIX, made on the base of writings dating XVI century or earlier). And here is what I found.
The dictionary tells that the preposition въ could be used to designate that something happened in some time, and here comes one of the examples:
А татя в первые продати противу поличного, а въ другiе уличатъ, продадутъ его не жалуя, а уличатъ въ третьие ино повѣсити. Уставная грамота великого князя Василия Дмитриевича двинянам, 1397г.
Since it is an example from a preposition's entry, I think that "впервые" might have developed from a combination of the preposition "въ" and the noun "пьрво".
I haven't found any noun "пьрво" in the dictionary, there was only an adverb "пьрво = перво" that meant "раньше, прежде". However, I've found another example in the entry about "въ". I believe, "пьрво" acts as a noun there:
Въ том градѣ иже в то перво столникъ и игуменъ Iеванъ въсхотѣ сътворити гистерноу. Житие преподобного Федора, игумена Сикейского монастыря, XVI век.
However, it is not clear what is the meaning of "перво" here. Something like "(earlier) time", maybe. Or it might be just the first part of the word "первостолникъ", written separately. But in this case I don't know how to translate "въ то". Can it be "тогда"?