First flight covers were quite common (and for a time, the rage) from the twenties through the sixties, not only for domestic flights but also other types such as you have. My comment on an earlier thread concerning the addressees:
Quote:
this is very common with first flight covers, of which I have maybe 1,000-1,200. When a first flight date was announced, collectors from different countries would send pre-addressed franked envelopes to the post master at the originating city. The postmaster at that location would stamp the cachet on the envelope and cancell the postage when the flight was ready to go. At the destination, the post master would dump the envelopes into the local PO and the cover mailed via regular mail to the addressee. Many times the post master at the originating city would sign random envelopes. I have quite a few with signatures also
Having said that, ffc's for doestic flights are certainly the most common.
Concerning your covers, they are very nice covers and certainly collectable. Most likely the insert in the one cover is just an insert to stiffen the cover. I have covers with cutout pieces of cardboard from all kinds of sources. I would not disturb the envelope.