API VKontakte. What are non-existent posts with post_id of 10 digits?

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I request posts from the wall via the VKontakte API.
In the results, among normal posts, 10-15 percent of posts come with a 10-digit post_id (for example, 1641934800). Moreover, the source_id (i.e., the id of the group where the post came from) is different for everyone, but this big number is the same everywhere. If you try to open this post in the browser, then VK writes: Error. Record not found.
I assume that these are advertising posts, but I would like to know for sure.