Was there ever a time where 'enclosure' was used as an alternative to 'attachments' of an e-mail?

Solution 1:

FEMA Instruction 5400.4 of 15 March 2000 has

4. Documentation Requirements for E-Mail Messages That Are Records. Hard copy printouts of E-mail records must include all pertinent identifiers that make the message intelligible.

   ...

   b. Additional Requirements. When necessary for complete documentation, print out:

   1. Enclosures. Print out enclosures only once if the same enclosure is being sent to multiple addresses.

So the term was in use in the US Federal Government in 2000. That document supersedes Temporary Directive 96-01(T), Electronic Mail Recordkeeping Guidance dated 27 March 1996, and it's possible that the term appears that early. However the earlier document does not appear to be available online to check.