I'm writing this to show an easy way to test your REST API calls. This brief video shows you how to go about it. In the video we also answer a question about the Gradebook REST APIs. The question was whether the letter grade shows up in the REST call results when the letter grade is secondary. Watch to the video to find out, and learn how you can quickly troubleshoot REST API calls! https://www.screencast.com/t/xOjJowsP4JH
Update: We brought this to the attention of the development team with JIRA ticket API-1084. They've been clear in stating this is functioning as designed. Should you require different behavior, bring it to our attention on https://community.blackboard.com/community/developers/rest/pages/rest-api-ideas.