In modern writing about legal affairs, it is common to say something along the lines of:
The judge heard his guilty plea.
He decided to plead guilty.
It seems to me like in this legal sense of the words, "plead" is used as a verb while "plea" is used as a noun, even in a context where both represent the same concept.
As a simple example, consider this opening sentence from Wikipedia's page on "plea bargain," which uses "plead" as a verb, almost as if it were an inflection of the same word.
The plea bargain (also plea agreement, plea deal, copping a plea, or plea in mitigation) is any agreement in a criminal case between the prosecutor and defendant whereby the defendant agrees to plead guilty to a particular charge in return for some concession from the prosecutor.
Yet according to the OED, the two words have totally separate etymologies, and both words have a noun form and verb form.
Plea: < Anglo-Norman and Old French plaid (842 in Old French in the Strasbourg Oaths in sense ‘pact, agreement’; French plaid ; compare α forms), Anglo-Norman pleet, pleit, plete,
Plead: < Anglo-Norman plaider, plaidier, pleder, pledire, pledre, pleider, pleidier and Old French pledeer, pledier, Old French, Middle French plaidier
Have these words, despite their separate etymologies, always been used in a somewhat interchangeable way, as in the Wikipedia quote? Or is that a more recent phenomenon resulting from the similarity of the words?
Edit: It seems people are either finding this question unclear or disliking it. Let me add a clarification that I also posted as a comment.
I understand that the words are not exactly used "interchangeably." My point is that both "plea" and "plead" are both nouns and verbs, and neither word derived from the other in English. So hypothetically, one could say:
I plea guilty.
He gave a guilty plead.
But today, it appears we always use "plead" as a verb and "plea" as a noun, often both referring to the notion of:
a legal plea where one pleads guilty or not guilty.
My question is why that is and for how long.