The Soviet Union was constantly testing improvements and modifications to the Soyuz spacecraft in both piloted and unpiloted mode. Although Cosmos 638 may have had connections with ASTP, it could have been testing a series of new systems so there is no certainty in the connection being a close one.

For one thing, it flew a longer mission - ten as opposed to six days, and for another it did not orbit at the same height as the ASTP craft. For completeness, highlights of its mission are listed here.