This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
While the contents of a written instrument should be proved by the introduction of the instrument itself in evidence, if the party against whom such evidence is offered insists upon such evidence, this is not due to the so-called parol evidence rule, but to the best evidence rule. If extrinsic evidence of the contents of a written instrument can be offered without violating the parol evidence rule, as where it is shown the written instrument is lost, such evidence is admissible as far as the parol evidence rule is concerned.1 Oral evidence of the contents of a lost notice may be given.2 In an action against a telegraph company for refusal to transmit a telegram, oral evidence of the contents of such telegram may be offered.3 The loss of one of two receipts in duplicate does not affect the legal operation of such receipt, and extrinsic evidence of the loss and contents of such receipt is not a violation of the parol evidence rule.4 Oral evidence is admissible to contradict such secondary evidence as to the contents of the lost written instrument.5
However, such evidence must always be limited to the contents of the written instrument. Other extrinsic evidence is governed by the rules that would be applicable if the written instrument were in evidence. If the written instrument supposed to be lost is found during trial, further evidence of its contents is inadmissible, even if some evidence has already been introduced.6