Polymorphic JSON property with type descriminator in same model?

[just-ero]

I have these models:

record Content(int TimeStamp);
record MessageContent(int TimeStamp, string Body) : Content(TimeStamp);

record Response(string ContentType, Content Content);

I need to be able to deserialize a response like this:

{
  "ContentType": "t.message",
  "Content": {
    "TimeStamp": 0,
    "Body": ""
  }
}

I expect an instance of Response where Content is an instance of MessageContent.

How can I do this with System.Text.Json? If I need a converter or a converter factory, what would that look like?