In fact there were seat belts and three point seat belts slightly before, even one patented in the mid fifties in USA. But there was a lot of variation. The early ones did not have reels, but what was the latch like? Do you adjust near the top or near the lap? Are there two buckles or one? In fact Volvo went through three pretty different styles before adopting the more modern sort that spool - the one in the image, the one right after that in the later PVs, 120, and 220 series that had the red levers, and the one in 140 and later Amazons with the hoops. So it was sort of a moot point really. Auto manufacturers would not have much trouble at all to come-up with their own three point belts that were different enough, and really they were all pretty different than those Volvo style front ones, some early Mercedes ones were similar in latch but different in many respects too. What was unique about Volvo was that they had a study to see if they were efficacious and how quickly they adopted them because of the leadership and goals of the company at the time. Their promise that this was an open patent could be seen as part of that.
/edit: full disclosure, happy Volvo owner: https://lh3.googleusercontent.com/-rasUCODYVHc/UHQj2hJsoDI/A...