Envelop has just lost the terminal "e" over time. See OED. The terminal "e" was still common in the latter 19th C., thus it was still used by older people in the early 20th C.
The OED's comment on ON-velope is pertinent. "...still very frequently heard although there is no good reason for giving a foreign sound to a word which no one regards as alien and which has been anglicized in spelling for nearly 200 years."