This is based on a situation in a mystery novel. In the novel a lawyer receives an envelope addressed to him by a client, in the client's handwriting, which the lawyer knows well. But the letter inside is not addressed to the lawyer, or indeed to anyone. Enclosed are sheets of paper with a watermark of the sort used in printing paper money, strongly suggesting a forgery operation. The unaddressed and unsigned note is consistent with the idea of a forgery operation. The note is also in the client's distinctive handwriting.
The lawyer assumes that the contents of the envelope were placed there by a mistake of the client, and were intended to be sent to a confederate of the client in this illegal scheme. He further assumes that a letter intended for the lawyer was sent to the confederate.
The lawyer had not previously known or suspected that the client was engaged in forgery or any unlawful activity.
The lawyer decides to treat this as a confidential and privileged matter. But the narrator remarks that he is obviously wrong to do so. One passage reads:
The obvious inference was that the letter which had come to Penfield [the lawyer] contained incriminating matter. That would explain everything. For if Penfield had thus stumbled on evidence of a crime, either committed or contemplated, he would have to choose between denouncing the criminal or keeping the matter to himself. But he was not entitled to keep it to himself; for, other considerations apart, this was not properly a client's secret. It had not been communicated to him: he had discovered it by accident. He was therefore not bound to secrecy, and he could not, consequently, claim a lawyer's privilege. In short, if he had discovered a crime and chose to suppress his discovery, he was, in effect, an accessory, before or after the fact, as the case might be; and he would necessarily keep the secret because he would not dare to divulge it.
(Source: Chapter IV of The Shadow of the Wolf by R. Austin Freeman, first published in 1925, and now out of copyright. Freeman died in 1943. See http://gutenberg.net.au/ebooks05/0500491h.html)
The novel is set in England, roughly in 1925.
My question is, would the contents of the envelope be considered privileged? Would the lawyer be required to divulge it if asked by the police or in a court? Would he be required to report the matter to the police on his own? What difference, if any does it make that the client apparently did not intend to send these contents to the lawyer? Are the answers different in 1925 England, 1925 US (say New York) 2020 England, or 2020 New York?
I know that a lawyer cannot regard as privileged advice on how to commit a crime, or plans for a future crime. I understand that a statement by a client that the client has committed a crime in the past is privileged. But what about this sort of unintended disclosure by the client?