According to this page on the website of a company named “Applied Fire Technologies, LLC”, Masterguard, supposedly a brand name/“sub-brand” of the Gentex Corporation that they sold smoke detectors, heat detectors, CO detectors, & fire extinguishers to residential consumers with, is no more. When exactly is unknown, but going by the fact that Gentex discontinued the last smoke detectors that Masterguard were selling a year ago or so, it’s likely recent. As that page says Applied Fire has taken over customer service for the Masterguard brand & had masterguard.com redirect to that page on their website (the latter of which is more evidence that Masterguard is gone as I know it still worked only some years back). I also believe that Applied Fire was once a “customer” of Masterguard (or rather Sure Signal Products, the supposed original maker of those wind-up heat detectors) so-to-speak as they sold/sell a version of Masterguard’s QR50 heat detector (along with several other companies as this page shows: “ThermaLINK” was/is Applied’s model), so I guess it would make sense that they would “take over operations” for what’s left of Masterguard.