"BEL.B" likely stands for Belgium Browning, "7.65" being the caliber (7.65×17mm Browning/32 ACP), with "74490" being the serial number. It was likely a Browning
M1922, a variant of the
M1910 with a longer barrel and magazine, which was made for military use including for the Germans after they took over Belgium. It was also produced chambered for the 9x17mm Browning/9mm kurz (short)/380 ACP. The M1922 link lists the multiple variations of that model number to help identify it. It also shows that the serial number is within the 1940-1944 production time frame. The 32 auto was a much more popular military and police cartridge in Europe than it was here where the 380 auto/9mm short was preferred. The 9×19mm Parabellum/9mm luger is a different cartridge that would not match the paperwork.