I just checked the Marketplace and there are now 26 Redwing 1935 airports available, so it looks like the backlog has been cleared.
The airmail-capable airports per the Marketplace descriptions include:
- KABE - Allentown/Bethlehem/Easton
- KBEI - Bellefonte
- KBWI - Baltimore Logan
- KCLE - Cleveland
- KDC1 - Hoover
- KEWR - Newark Liberty
- KLGA - LaGuardia
- KMDW - Chicago Midway