For how long has "plead" been used as a verb counterpart to "plea" (noun) even though both words historically have both verb and noun forms?

Question

In modern writing about legal affairs, it is common to say something along the lines of:

The judge heard his guilty plea.

and

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?

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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.

Or

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.

Answer 1

According to the following source the use of plea as a verb is a regionalism, while plead as a noun is just a non-standard usage:

In general use, the verb plead means

• to make an earnest appeal, entreaty, or supplication; to beg, implore.

The noun plea has similar legal and general meanings:

plea: - 1. A suit or action at law; the presentation of an action in court. An urgent, emotional request, an entreaty; (also) an unarticulated appeal.

Used as a verb in place of plead, plea can be regarded as a regionalism (Chiefly Eng. regional [north.], and Sc. Now also U.S.):

• If you plea guilty and then later in another hearing say th(at you didn’t do it, can you be charged with perjury? –example of U.S. usage in OED

Using the verb form plead for the noun plea, however, is jarringly nonstandard:

• A Plead to Sinners –title of a poem on a religious site

• A plead to ban homophobia –headline on a college site

• A plead for help –part of a blog title

In each of these examples, the word wanted is plea.

Also from blog.bennettandbennett.com:

When a person admits her guilt in court, she does not plea guilty. She pleads guilty, entering a guilty plea. “Plead” is a verb. “Plea” is a noun.

After a person has admitted his guilt in court, he has not plead (or “pleaed”) guilty. The past tense of “to plead” is “pleaded” or “pled.”

So, referring to regional, non-standard usages, most likely due to the similarity of the spelling of the two terms, it is really hard to find a precise period when it all started. Google Books, if it can be of any real help here, suggest usages of a plead for and plea guilty from the mid-20th century.