The second flap is devised to be free (not attached to anything), see:

Can This Be Right?!
Can This Be Right?!

Added in the early 60ies and some documents describing the operation of pneumatically governed IP pumps show only one flap in the intake(like here):
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/615/07-010.pdf