Greece 1966 Express letter to London England, with red boxed EXPRES (presumably Greek) and violet boxed EXPRESS FEE PAID (British).
Two stamps, SG 770 & 785 pay the 11d50 postage.
On the reverse:
ATHINAI-AVION 1 IV 66 21 (continuous machine postmark) and violet London EC. 2 APR 66
Roughly opened at top.