Just a day after rolling out a new Windows 10 preview build for PCs, Microsoft is making another new test build -- with 300 fixes -- available to its Insider testers. By Mary Jo Foley for All About Microsoft | June 30, 2015 -- 21:20 GMT (14:20 PDT) | Just one day after releasing a new Windows 10 preview build for PCs, Microsoft is rolling out another test build. On June 30, Microsoft is making available to its Windows Insiders on the Fast Ring preview Build 10159. Yesterday, Microsoft released Build 10158 to its Fast Ring testers. Why is Microsoft releasing two test builds back-to-back -- deviating from the usual pace of a new test build every few weeks? For one, the company is closing in on designating a Windows 10 for PC build as "release to manufacturing." Microsoft is interested in getting its millions of Insiders to test the many bug fixes and final feature tweaks before RTM. Today's build includes more than 300 bug fixes, according to company officials. According to Gabe Aul, head of the Windows Insider program, the other reason was because 10159 was deemed stable enough to go out to testers. And it includes one new "surprise" feature -- which may or may not be the inclusion of the new official Windows 10 "hero image" wallpaper, a screen shot of which is embedded above in my blog post. (Aul wouldn't say for sure in his post or on Twitter.) From Aul's June 30 post: "Why two builds so fast back to back?! As you probably know, we validate builds that we produce in internal rings where they're used by engineers in OSG (Operating Systems Group) and Microsoft. If they pass all of our criteria we make them available to Windows Insiders, first to Fast and then to Slow once we know it's a stable build. We'd been using 10158 for almost a week and knew it was going to be a strong build, and the data backed that up so we released it yesterday. Hot on its heels though was build 10159, which in addition to more fixes (over 300 of them!), took in one very interesting change. We didn't want to wait with 10158 so we went ahead and released that build while we were still evaluating 10159. As it turns out though, 10159 is also a great build and passes our criteria for the Windows Insider Fast ring." Build 10159 will start rolling out later today to Insiders, and be available to all by end of day (meaning Pacific time, U.S., I assume), Aul said.