The V-Mail was for forces on active duty during WWII. To save space, soldiers mail was photographed & the resulting films (taking up much less space in the planes) were posted back to central offices where they were developed, printed & then mailed out in those special window envelopes. Make sure you keep the envelopes with the photo/letter, collectors like to show both.
A more comprehensive article on Wiki here -https://en.wikipedia.org/wiki/V-mail
And the Smithsonian here -https://postalmuseum.si.edu/exhibit.../v-mail.html