Apollo-Soyuz Test Project (ASTP) received a lot of philatelic attention as not only being the first ever joint space flight but also as the last Apollo flight and the last splashdown event.
Many stamps and covers were issued officially and privately including cinderellas. I have some in my collection and will be glad to see others that I do not have.
I will begin from the probably most famous space "non-postal" stamps creator - NASA local post. In 1967, Space City Cover Society (SCCS) established its own Local Post under the name – NASA Local Post (NLP). There has been no connection to NASA or any other government agency, just a nice logo.
NLP produced special ASTP cancellation and 2 types of cinderellas - "emblem" and "docking". Actually there were 3 types of cinderellas - if you look at the "docking" sheet - there are 7 Soyuz-Apollo stickers and 2 inverted - Apollo Soyuz (in the left upper corner). Four different covers were used for NASA Local Post with four colors of postmarks, though the colors were assigned to a specific event, sometimes you can find "wrong" color on "wrong" cover.