Well, there might be a better sub for this...some collectors have memorized amazing amounts of detailed information about their collections, and one might stop by and expound.

But, no, there were other semi-auto pistols before 1950 that had staggered magazines. The Mauser C96 "Broomhandle" has staggered cartridges in the magazine, though that magazine wasn't detachable until the later models of the 1930's. There were plenty of those made. The Savage Model 1907 had both staggered rounds and a detachable magazine that held 10 rounds of .32 ACP. It was used by the French Army in WWI and it came out before the Browning High-Power. It probably should be put into that very small pile of " bright ideas for firearms design not originated with John Browning", but I know little about its history.

Why arms makers didn't have staggered-ammo detachable magazines more often for military pistols is a broad question. We could speculate intelligently, however. First, pistols were not important: if an officer had to pull out his pistol, it was either to lead troops who were armed with rifles, machine guns, etc. or something had gone very wrong, so magazine capacity was therefore kind of trivial. Second, there was a division of opinion about which was better for reliability and reloading, the newer semi-automatic pistols with spare magazines or the older revolvers with half-moon clips. George S Patton preferred revolvers, instead of the newer 1911A1 , perhaps because when he was a young officer he used the standard M1917 , and that of course was a "six shooter". The M1917 service revolver saw service in WWII as well. Last, from a design standpoint, many semiautomatic pistol designs ( like Lugers) were already plagued by ammo mis-loading , and large capacity magazines had more problems with feeding- it is very hard to have a magazine spring that exerts as much force fully compressed as it does fully expanded ( nor was the problem limited to pistols: the British had a lot of trouble getting their Sten gun magazines to feed properly). The designers might easily have felt that 8 reliable shots was better than 10 possible, and still better than 6 in a revolver.