The latest version of Inbox, which is v1.7, was recently torn down by the folks over at Android Police, as usual. Typically in these tear downs they find some pretty interesting strings of code that relate to upcoming features. And it's no different this time around. The latest version of Inbox is v1.7 and has been available for a few days (download link is at the bottom of the post).
Android Police has torn apart the APK and found out that custom email signatures on their way. As there is support for them in this update to Inbox. While email signatures aren't available right now, they will be soon. As the code is already available in the APK. It appears, from the code at least, that each signed in email account will be able to have its own signature. Which is a good thing. Especially if you have your work and personal email in Inbox.
While the code is there, it doesn't tell us how close the feature is to being available on Inbox. It could be tomorrow, it could be next year. It's also not known whether or not it just needs to be turned on, on the server side, or if there's another update coming soon to enable email signatures.
Outside of the email signatures, the APK appears to be just a bug fixer for Inbox by Gmail. Which is always good. There are always bugs that need to be fixed these days. While Google could be saving email signatures for Google I/O in a few weeks, I find it hard to believe, unless there are some other big features coming to Inbox. As many different products as Google has, I think they'd skip Inbox in the keynote, unless there's something big coming to Inbox.
As stated above, we do have the APK down below for you to download and sideload. Simply download the APK to your device. Go into Settings > Security and make sure "Unknown Sources" is checked and open the APK. It'll walk you through the install process and replace the current Inbox APK on your device.
Download: Inbox by Gmail v1.7