This is an Adam Bert Cover on a Number 5 Envelope - Unlisted
Adam Bert began servicing covers in 1923 and during the late 20s he
began using uncacheted small (number 5) envelopes (3" x 5 1/2"). Adam Bert has continued to service FDCs to the present time on these number 5 envelopes and in addition his covers usually contain his address on the envelope.