Thanks for your response. I now understand that
timestamp_ms is only a property of tweet objects from the streaming API. I believe the code that gathers the streaming data I was examining is inadvertently dropping the fields and with your response, I have leverage to compel an examination of that code.
When I query directly,
timestamp_ms is a key in results returned from
twurl -H stream.twitter.com -A 'Accept-encoding: none' -X POST '/1.1/statuses/sample.json'
timestamp_ms is not a key in results from
twurl 1.1/statuses/sample.json or the Tweepy examples I tested, but these are not streaming API, so this is expected.
If the documentation at https://developer.twitter.com/en/docs/tweets/sample-realtime/api-reference/get-statuses-sample had the Example Response filled out, it would be helpful.
timestamp_ms was part of the the documentation at https://developer.twitter.com/en/docs/tweets/data-dictionary/overview/intro-to-tweet-json, I could have answered my own question.