Thanks for sharing. I have a large collection of US airmail FDCs, and like to have as many of them with plate blocks as possible (or panes in the case of booklet stamps, and line pairs in the case of coil issues). I had not noticed those inserts before, so I went and got my album with the Hawaii 80 cent issue and found them in the following Artmaster cacheted cover:

Looking to see if I had a Fleetwood cacheted cover (like yours), I did find one (three, actually) but not with those inserts. And I found this:

Note that the Fleetwood cachet in this one has been tinted. Curiously, the album page at the bottom right has the Artmaster insignia. I've not seen other Armaster album pages like this. Hand tinting like this is often one-off, so this Fleetmaster cover on an Artmaster album page may be one of a kind.
I have a lot of other FDCs of this issue, as I do for most US airmail stamps. In fact, I have FDCs for all 150 US airmail stamps, but just one each, with singles, of C1-C6. After that, I begin to have FDCs with plate blocks (except for the Zepps! -- though I did acquire FDCs of the Zepps with singles last year).
Basil