Tiny translucent glassine envelopes. Glassine is archival and is normally used for preserving photographic negatives. These have no adhesive.
1-3/4" x 2-3/4" or 2" x 2".
The 2x2 size are also included in the Vintage Voyage Kit; see all the other kit components available to buy separately.
ATC-sized glassine envelopes are below.
Sets of 10 envelopes (rectangular are sold out).